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adding optional records

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Luke Boswell 2023-02-08 18:49:08 +11:00
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www/generate_tutorial/src/input/tutorial.md
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@ -400,7 +400,47 @@ Note that `&` can't introduce new fields to a record, or change the types of exi
## [Optional Record Fields](#optional-record-fields) {#optional-record-fields}
\[This part of the tutorial has not been written yet. Coming soon!\]
Roc supports optional record fields using the `?` operator. This can be a useful pattern where you pass a function a record of configuration values, some of which you don't really care about and want to leave as defaults.
In Roc you can write a function like;
<pre><samp>table <span class="kw">=</span> <span class="kw">\</span><span class="paren">{</span> height, width, title <span class="colon">?</span> "", description <span class="colon">?</span> "" <span class="paren">}</span> <span class="kw">-></span>
</pre></samp>
This is using *optional field destructuring* to destructure a record while
also providing default values for any fields that might be missing.
Here's the type of `table`;
<pre><samp>table <span class="colon">:</span>
<span class="paren">{</span>
height <span class="colon">:</span> Pixels,
width <span class="colon">:</span> Pixels,
title <span class="colon">?</span> Str,
description <span class="colon">?</span> Str,
<span class="paren">}</span>
<span class="kw">-></span> Table
table <span class="kw">= \</span><span class="paren">{</span> height, width, title <span class="colon">?</span> "", description <span class="colon">?</span> "" } <span class="kw">-></span>
</pre></samp>
This says that `table` takes a record with two *required* fields, `height` and
`width`, and two *optional* fields, `title` and `description`. It also says that
the `height` and `width` fields have the type `Pixels`, a type alias for some
numeric type, and the `title` and `description` fields have the type `Str`.
This means you can choose to omit the `title`, `description`, or both fields, when calling the function... but if you provide them, they must have the type `Str`.
This is also the type that would have been inferred for `table` if no annotation
had been written. Roc's compiler can tell from the destructuring syntax
`title ? ""` that `title` is an optional field, and that it has the type `Str`.
These default values can reference other expressions in the record destructure; if you wanted, you could write `{ height, width, title ? "", description ? Str.concat "A table called " title }`.
Destructuring is the only way to implement a record with optional fields. For example, if you write the expression `config.title` and `title` is an
optional field, you'll get a compile error.
This means it's never possible to end up with an *optional value* that exists
outside a record field. Optionality is a concept that exists only in record
fields, and it's intended for the use case of config records like this. The
ergonomics of destructuring mean this wouldn't be a good fit for data modeling.
## [Tags](#tags) {#tags}