builtin(str): add withPrefix

Closes #4142

Signed-off-by: Prajwal S N <prajwalnadig21@gmail.com>

crates/compiler/builtins/roc/Str.roc

@@ -43,6 +43,7 @@ interface Str
         appendScalar,
         walkScalars,
         walkScalarsUntil,
+        withPrefix,
     ]
     imports [
         Bool.{ Bool },
@@ -561,3 +562,7 @@ strToNumHelp = \string ->
         Ok result.aresult
     else
         Err InvalidNumStr
+
+## Adds the specified prefix to the string, like a reversed Str.concat
+withPrefix : Str, Str -> Str
+withPrefix = \str, prefix -> Str.concat prefix str

crates/compiler/module/src/symbol.rs

@@ -1296,6 +1296,7 @@ define_builtins! {
         50 STR_REPLACE_EACH: "replaceEach"
         51 STR_REPLACE_FIRST: "replaceFirst"
         52 STR_REPLACE_LAST: "replaceLast"
+        53 STR_WITH_PREFIX: "withPrefix"
     }
     6 LIST: "List" => {
         0 LIST_LIST: "List" exposed_apply_type=true // the List.List type alias

crates/compiler/test_gen/src/gen_str.rs

@@ -1930,3 +1930,27 @@ fn when_on_strings() {
         i64
     );
 }
+
+#[test]
+#[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
+fn str_with_prefix() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+            Str.withPrefix "world!" "Hello "
+            "#
+        ),
+        RocStr::from("Hello world!"),
+        RocStr
+    );
+
+    assert_evals_to!(
+        indoc!(
+            r#"
+            "two" |> Str.withPrefix "Forty "
+            "#
+        ),
+        RocStr::from("Forty two"),
+        RocStr
+    );
+}