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<title data-rh="true">Code Actions (a.k.a. Assists) | ELP</title><meta data-rh="true" name="viewport" content="width=device-width,initial-scale=1"><meta data-rh="true" name="twitter:card" content="summary_large_image"><meta data-rh="true" property="og:url" content="https://whatsapp.github.io/erlang-language-platform/docs/contributing/code-actions/"><meta data-rh="true" name="docusaurus_locale" content="en"><meta data-rh="true" name="docsearch:language" content="en"><meta data-rh="true" name="docusaurus_version" content="current"><meta data-rh="true" name="docusaurus_tag" content="docs-default-current"><meta data-rh="true" name="docsearch:version" content="current"><meta data-rh="true" name="docsearch:docusaurus_tag" content="docs-default-current"><meta data-rh="true" property="og:title" content="Code Actions (a.k.a. Assists) | ELP"><meta data-rh="true" name="description" content="Code actions, also known as assists, are small local refactorings, often rendered in the text editor using a light bulb icon (💡). They are triggered by either clicking the light bulb icon in the editor or by using a shortcut."><meta data-rh="true" property="og:description" content="Code actions, also known as assists, are small local refactorings, often rendered in the text editor using a light bulb icon (💡). They are triggered by either clicking the light bulb icon in the editor or by using a shortcut."><link data-rh="true" rel="icon" href="/erlang-language-platform/img/elp_icon_color.svg"><link data-rh="true" rel="canonical" href="https://whatsapp.github.io/erlang-language-platform/docs/contributing/code-actions/"><link data-rh="true" rel="alternate" href="https://whatsapp.github.io/erlang-language-platform/docs/contributing/code-actions/" hreflang="en"><link data-rh="true" rel="alternate" href="https://whatsapp.github.io/erlang-language-platform/docs/contributing/code-actions/" hreflang="x-default"><link rel="stylesheet" href="/erlang-language-platform/assets/css/styles.93dc32ac.css">
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Assists)</span><meta itemprop="position" content="2"></li></ul></nav><div class="tocCollapsible_ETCw theme-doc-toc-mobile tocMobile_ITEo"><button type="button" class="clean-btn tocCollapsibleButton_TO0P">On this page</button></div><div class="theme-doc-markdown markdown"><h1>Code Actions (a.k.a. Assists)</h1><p><em>Code actions</em>, also known as <em>assists</em>, are small local refactorings, often rendered in the text editor using a light bulb icon (💡). They are triggered by either clicking the light bulb icon in the editor or by using a shortcut.</p><p>Code actions often provide the user with possible corrective actions right next to an error or warning (known as a <em>diagnostic</em> message using LSP jargon). They can also occur independently of diagnostics.</p><p>Here is an example of a <em>code action</em> prompting the user to <em>add an EDoc comment</em> for a function which lacks Erlang EDoc documentation.</p><p><img loading="lazy" alt="Code Action - Add Edoc" src="/erlang-language-platform/assets/images/code-action-add-edoc-00c08bb4c5326a4f4544e606890fd667.png" width="968" height="376" class="img_ev3q"></p><p>And this is what the code looks like after the suggestion has been applied:</p><p><img loading="lazy" alt="Code Action - Add Edoc Fix" src="/erlang-language-platform/assets/images/code-action-add-edoc-fix-41b68bc5f8f0af57dd2dcdc686e0d850.png" width="1502" height="326" class="img_ev3q"></p><h2 class="anchor anchorWithStickyNavbar_LWe7" id="the-code-action-request">The <em>Code Action</em> request<a href="#the-code-action-request" class="hash-link" aria-label="Direct link to the-code-action-request" title="Direct link to the-code-action-request"></a></h2><p>Code actions are requested by the editor using the <a href="https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textDocument_codeAction" target="_blank" rel="noopener noreferrer">textDocument/codeAction</a> LSP request. Code action requests are handled by the <code>handlers::handle_code_action</code> function in the <code>elp</code> crate.</p><h2 class="anchor anchorWithStickyNavbar_LWe7" id="adding-a-new-code-action">Adding a new code action<a href="#adding-a-new-code-action" class="hash-link" aria-label="Direct link to Adding a new code action" title="Direct link to Adding a new code action"></a></h2><p>In this section we will go through the process of adding a new code action from scratch. The code action (or <em>assist</em>) will suggest the user to delete a function, if it is deemed as unused by the Erlang compiler.</p><h3 class="anchor anchorWithStickyNavbar_LWe7" id="creating-the-handler">Creating the handler<a href="#creating-the-handler" class="hash-link" aria-label="Direct link to Creating the handler" title="Direct link to Creating the handler"></a></h3><p>Let's start by creating a new file named <code>delete_function.rs</code>, containing a single function declaration:</p><div class="language-rust codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockTitle_Ktv7">crates/ide_assists/src/handlers/delete_function.rs</div><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-rust codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token plain">use crate::assist_context::{Assists, AssistContext};</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">pub(crate) fn delete_function(acc: &mut Assists, ctx: &AssistContext) -> Option<()> {</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> todo!()</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">}</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div><p>Before we can start implementing our code action, there's one more thing we need to do: ensure our new function is invoked by adding it to the list of <em>ELP assists</em>. Open the <code>crates/ide_assists/src/lib.rs</code> file and amend the list of handlers:</p><div class="language-rust codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockTitle_Ktv7">crates/ide_assists/src/lib.rs</div><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-rust codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token plain">mod handlers {</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> [...]</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> mod delete_function</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> [...]</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> pub(crate) fn all() -> &'static [Handler] {</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> &[</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> [...]</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> delete_function:delete_function,</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> [...]</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> ]</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> }</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">}</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div><h3 class="anchor anchorWithStickyNavbar_LWe7" id="adding-a-test-case">Adding a test case<a href="#adding-a-test-case" class="hash-link" aria-label="Direct link to Adding a test case" title="Direct link to Adding a test case"></a></h3><p>The easiest way to verify our new code action behaves in the expected way is to start with a test case. ELP allows us to write tests in a very intuitive and straightforward way.</p><p>Add the following to the <code>delete_function.rs</code> file:</p><div class="language-rust codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockTitle_Ktv7">crates/ide_assists/src/handlers/delete_function.rs</div><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-rust codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token plain">#[cfg(test)]</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">mod tests {</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> use expect_test::expect;</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> use super::*;</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> use crate::tests::*;</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> #[test]</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> fn test_delete_unused_function() {</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> check_assist(</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> delete_function,</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> "Remove the unused function `heavy_calculations/1`",</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> r#"</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> -module(life).</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> heavy_cal~culations(X) -></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> %% ^^^^^^^^^^^^^^^^^^^ 💡 L1230: Function heavy_calculations/1 is unused</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> X.</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> meaning() -></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> 42.</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">"#,</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> expect![[</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> r#"</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> -module(life).</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> meaning() -></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> 42.</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> "#]],</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> )</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> }</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">}</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div><p>There is a lot happening here, so let's go through the code. We are defining a new test, named <code>test_delete_unused_function</code>, which uses an auxiliary function (<code>check_assist</code>) to verify that a given assist behaves as expected.</p><p>The <code>check_assist</code> function takes 4 arguments:</p><ul><li>The assist <em>handler</em> (<code>delete_function</code>)</li><li>A <em>label</em> for the assist</li><li>An input fixture representing what the code looks like <strong>before</strong> a fix is applied</li><li>An output fixture (wrapped in an <code>expect</code> macro) showing what the code looks like <strong>after</strong> a fix is applied</li></ul><p>The <code>~</code> in the first snippet represents the cursor position. We are asserting that - given a diagnostic message pointing to the unused function - if the user triggers the respective code action when the cursor is hovering the function name range, the unused function gets deleted.</p><p>Let's try running the test, it should fail with a <em>not yet implemented</em> error:</p><div class="codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-text codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token plain">$ cargo test --package elp_ide_assists --lib -- handlers::delete_function::tests::test_delete_unused_function --exact --nocapture</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">[...]</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">---- handlers::delete_function::tests::test_delete_unused_function stdout ----</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">thread 'handlers::delete_function::tests::test_delete_unused_function' panicked at 'not yet implemented', crates/ide_assists/src/handlers/delete_function.rs:21:5</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">[...]</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div><h3 class="anchor anchorWithStickyNavbar_LWe7" id="diagnostic-annotations-and-error-codes">Diagnostic Annotations and Error Codes<a href="#diagnostic-annotations-and-error-codes" class="hash-link" aria-label="Direct link to Diagnostic Annotations and Error Codes" title="Direct link to Diagnostic Annotations and Error Codes"></a></h3><p>Before starting with the actual implementation, let's for a second go back to the syntax we used to specify the <em>unused function</em> diagnostic:</p><div class="codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-text codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token plain">%% ^^^^^^^^^^^^^^^^^^^ 💡 L1230: Function heavy_calculations/1 is unused</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div><p>This is a test <em>annotation</em> which is used by the ELP testing framework to populate the "context" which is passed to our handler. This is a way to simulate diagnostics coming from external sources (such as the Erlang compiler or a linter), which would be received by the Language Server as part of a <code>textDocument/codeAction</code> request.</p><p>The annotation has the following format:</p><div class="codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-text codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token plain">[\s]%% [^]* 💡 CODE: MESSAGE</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div><p>Essentially, a number of spaces, followed by the <code>%%</code> which resembles an Erlang comment, a light bulb, a <em>code</em> identifying the diagnostic type and a string message. The <em>code</em> is an <em>unofficial error</em> code which is emitted by both ELP's <em>Erlang Service</em> (see the <code>erlang_service:make_code/2</code> function in <code>erlang_service/src/erlang_service.erl</code>) and by the <a href="https://github.com/erlang-ls/erlang_ls/" target="_blank" rel="noopener noreferrer">Erlang LS</a> language server. The idea is to eventually standardize Erlang error messages and to build what, in the end, should be similar to the <a href="https://doc.rust-lang.org/error-index.html" target="_blank" rel="noopener noreferrer">Rust</a> or <a href="https://errors.haskell.org/" target="_blank" rel="noopener noreferrer">Haskell</a> error indexes. In our case, <code>L1230</code> is the error corresponding to the <code>unused_function</code> diagnostic. The <em>message</em> is a free text string that accompanies the diagnostic.</p><h3 class="anchor anchorWithStickyNavbar_LWe7" id="matching-on-the-diagnostic-error-code">Matching on the diagnostic error code<a href="#matching-on-the-diagnostic-error-code" class="hash-link" aria-label="Direct link to Matching on the diagnostic error code" title="Direct link to Matching on the diagnostic error code"></a></h3><p>To be able to match the <code>L1230</code> error code, we need to add a new variant to the <code>AssistContextDiagnosticCode</code> enum. Open the <code>crates/ide_db/src/assists.rs</code> file and include the new error code. Don't forget to map it to the <code>L1230</code> string.</p><div class="language-rust codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockTitle_Ktv7">crates/ide_db/src/assists.rs</div><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-rust codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token plain">pub enum AssistContextDiagnosticCode {</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> UnusedFunction, // <--- Add this</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> [...]</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> UnusedVariable,</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">}</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">impl FromStr for AssistContextDiagnosticCode {</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> type Err = String;</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> fn from_str(s: &str) -> Result<Self, Self::Err> {</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> match s {</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> "L1230" => Ok(AssistContextDiagnosticCode::UnusedFunction), // <--- Add this</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> [...]</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> "L1268" => Ok(AssistContextDiagnosticCode::UnusedVariable),</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> unknown => Err(format!("Unknown AssistContextDiagnosticCode: '{unknown}'")),</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> }</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> }</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">}</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div><p>We are all set. Time to implement the <code>delete_function</code> function!</p><h3 class="anchor anchorWithStickyNavbar_LWe7" id="the-implementation">The implementation<a href="#the-implementation" class="hash-link" aria-label="Direct link to The implementation" title="Direct link to The implementation"></a></h3><p>Let's look at our function again.</p><div class="codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-text codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token plain">pub(crate) fn delete_function(acc: &mut Assists, ctx: &AssistContext) -> Option<()> {</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> todo!()</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">}</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div><p>We have two input arguments: a mutable <em>accumulator</em> which contains the list of code actions (or <em>assists</em>) which we want to return and a <em>context</em>, from which we can extract diagnostics.</p><p>The following code iterates through the list of diagnostics and, for each diagnostic matching the <code>UnusedFunction</code> kind, prints the diagnostic for debugging purposes. We return <code>Some(())</code> to comply with the function signature.</p><div class="codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-text codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token plain">use elp_ide_db::assists::AssistContextDiagnosticCode;</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">[...]</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">pub(crate) fn delete_function(acc: &mut Assists, ctx: &AssistContext) -> Option<()> {</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> for d in ctx.diagnostics {</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> if let AssistContextDiagnosticCode::UnusedFunction = d.code {</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> dbg!(d);</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> todo!()</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> }</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> }</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> Some(())</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">}</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">[...]</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block"></span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div><p>If we run the test, we can see what a diagnostic looks like:</p><div class="codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-text codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token plain">$ cargo test --package elp_ide_assists --lib -- handlers::delete_function::tests::test_delete_unused_function --exact --nocapture</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">[...]</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">running 1 test</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">[crates/ide_assists/src/handlers/delete_function.rs:25] d = AssistContextDiagnostic {</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> code: UnusedFunction,</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> message: "Function heavy_calculations/1 is unused",</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> range: 24..40,</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">}</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">[...]</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div><p>The diagnostic contains the error code and message, together with its range. What we want to do is:</p><ul><li>Find the function declaration which is pointed by the diagnostic range</li><li>Create a code action to remove the function declaration and add it to the accumulator</li></ul><p>How do we find the element which the range covers? Context to the rescue! There's a handy <code>find_node_at_custom_offset</code> function we can use. The <em>offset</em> here indicates the number of bytes from the beginning of the file. We can use the beginning of the diagnostic range for our purposes.</p><div class="codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-text codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token plain">let function_declaration: ast::FunDecl = ctx.find_node_at_custom_offset::<ast::FunDecl>(d.range.start())?;</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">let function_range = function_declaration.syntax().text_range();</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div><p>Let's extract the function name/arity and produce a nice message for the user:</p><div class="codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-text codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token plain">let function_name = function_declaration.name()?;</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">let function_arity = function_declaration.arity_value()?;</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">let message = format!("Remove the unused function `{function_name}/{function_arity}`");</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div><p>With the information we have, we can now create a new code action and add it to the accumulator:</p><div class="codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-text codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token plain">let id = AssistId("delete_function", AssistKind::QuickFix);</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">let function_range = function_declaration.syntax().text_range();</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">acc.add(id,</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> message,</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> function_range,</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> |builder| {</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> builder.edit_file(ctx.frange.file_id);</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> builder.delete(function_range)</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> },</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">);</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div><p>The <code>add</code> function takes four arguments:</p><ul><li>An internal <code>AssistId</code> made of a unique string (the <code>"delete_function"</code> string in our case) and a <code>Kind</code>. We are specifying <code>QuickFix</code> in our case, but have a look to the <a href="https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#codeActionKind" target="_blank" rel="noopener noreferrer">LSP specifications</a> to get a list of the available kinds.</li><li>A message which will be rendered to the user (<code>"Delete the unused function: [FUNCTION_NAME]"</code>)</li><li>The range of the function. Notice how the range we got from the diagnostic was covering only the <em>name</em> of the function, but we need to delete the whole function, so we need to pass the full range.</li><li>A function which takes a <code>builder</code> as its input and uses it to manipulate the source file. Here we are saying that we want to edit the current file (we extract the <code>file_id</code> from the <code>ctx</code> context) and that we simply want to delete the range of the function declaration.</li></ul><p>Yes. It's as simple as that. For completeness, here is the full function implementation:</p><div class="codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-text codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token plain">pub(crate) fn delete_function(acc: &mut Assists, ctx: &AssistContext) -> Option<()> {</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> for d in ctx.diagnostics {</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> if let AssistContextDiagnosticCode::UnusedFunction = d.code {</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> let function_declaration: ast::FunDecl =</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> ctx.find_node_at_custom_offset::<ast::FunDecl>(d.range.start())?;</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> let function_name = function_declaration.name()?;</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> let function_arity = function_declaration.arity_value()?;</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> let function_range = function_declaration.syntax().text_range();</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> let id = AssistId("delete_function", AssistKind::QuickFix);</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> let message = format!("Remove the unused function `{function_name}/{function_arity}`");</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> acc.add(id, message, function_range, |builder| {</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> builder.edit_file(ctx.frange.file_id);</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> builder.delete(function_range)</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> });</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> }</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> }</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> Some(())</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">}</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div><p>You can look at existing assists for more complex manipulation examples.</p><h1>Try it yourself</h1><p>What we wrote is a unit test, but there's nothing better than checking ourselves the behaviour in the IDE.</p><p>Compile the <code>elp</code> executable:</p><div class="codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-text codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token plain">cargo build</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div><p>Then visit the Erlang extension settings page and edit the <code>elp.path</code> value to point to the newly built executable, which should reside in:</p><div class="codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-text codeBlock_bY9V thin-scrollbar"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token plain">./debug/elp</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div><p>Open VS Code (or reload the window if you have it already open) and visit an Erlang file which contains an unused function. You should see something like:</p><p><img loading="lazy" alt="Code Action - Remove Function" src="/erlang-language-platform/assets/images/code-action-remove-function-17500a18e85501d12d483dc13df76153.png" width="880" height="467" class="img_ev3q"></p><p>If that worked, congratulations! You managed to write your first ELP code action!</p></div></article><nav class="pagination-nav docusaurus-mt-lg" aria-label="Docs pages"><a class="pagination-nav__link pagination-nav__link--prev" href="/erlang-language-platform/docs/contributing/"><div class="pagination-nav__sublabel">Previous</div><div class="pagination-nav__label">Contributing</div></a><a class="pagination-nav__link pagination-nav__link--next" href="/erlang-language-platform/docs/architecture/"><div class="pagination-nav__sublabel">Next</div><div class="pagination-nav__label">Architecture</div></a></nav></div></div><div class="col col--3"><div class="tableOfContents_bqdL thin-scrollbar theme-doc-toc-desktop"><ul class="table-of-contents table-of-contents__left-border"><li><a href="#the-code-action-request" class="table-of-contents__link toc-highlight">The <em>Code Action</em> request</a></li><li><a href="#adding-a-new-code-action" class="table-of-contents__link toc-highlight">Adding a new code action</a><ul><li><a 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