add helpful error message

CHANGELOG.md

@@ -239,6 +239,38 @@
 - Improved error title when using an unknown module value.
   ([Giacomo Cavalieri](https://github.com/giacomocavalieri))
 
+- The compiler now shows an helpful error message if you try writing an `if`
+  expression instead of a case. For example, this code:
+
+  ```gleam
+  pub fn main() {
+    let a = if wibble {
+      1
+    }
+  }
+  ```
+
+  Results in the following error:
+
+  ```txt
+  error: Syntax error
+    ┌─ /src/parse/error.gleam:3:11
+    │
+  3 │   let a = if wibble {
+    │           ^^ Gleam doesn't have if expressions
+
+  If you want to write a conditional expression you can use a `case`:
+
+      case condition {
+        True -> todo
+        False -> todo
+      }
+
+  See: https://tour.gleam.run/flow-control/case-expressions/
+  ```
+
+  ([Giacomo Cavalieri](https://github.com/giacomocavalieri))
+
 ### Formatter
 
 - The formatter now adds a `todo` after a `use` expression if it is the last

compiler-core/src/parse.rs

@@ -715,6 +715,12 @@ where
                 }
             }
 
+            // Helpful error if trying to write an if expression instead of a
+            // case.
+            Some((start, Token::If, end)) => {
+                return parse_error(ParseErrorType::IfExpression, SrcSpan { start, end });
+            }
+
             // helpful error on possibly trying to group with ""
             Some((start, Token::LeftParen, _)) => {
                 return parse_error(ParseErrorType::ExprLparStart, SrcSpan { start, end: start });

compiler-core/src/parse/error.rs

@@ -295,6 +295,19 @@ utf16_codepoint, utf32_codepoint, signed, unsigned, big, little, native, size, u
                 "Unsupported expression",
                 vec!["Functions cannot be called in clause guards.".into()],
             ),
+            ParseErrorType::IfExpression => (
+                "Gleam doesn't have if expressions",
+                vec![
+                    "If you want to write a conditional expression you can use a `case`:".into(),
+                    "".into(),
+                    "    case condition {".into(),
+                    "      True -> todo".into(),
+                    "      False -> todo".into(),
+                    "    }".into(),
+                    "".into(),
+                    "See: https://tour.gleam.run/flow-control/case-expressions/".into(),
+                ],
+            ),
         }
     }
 }
@@ -361,6 +374,7 @@ pub enum ParseErrorType {
         field_type: Option<TypeAst>,
     },
     CallInClauseGuard, // case x { _ if f() -> 1 }
+    IfExpression,
 }
 
 impl LexicalError {

compiler-core/src/parse/snapshots/gleam_core__parse__tests__if_like_expression.snap

@@ -0,0 +1,18 @@
+---
+source: compiler-core/src/parse/tests.rs
+expression: "\npub fn main() {\n  let a = if wibble {\n    wobble\n  }\n}\n"
+---
+error: Syntax error
+  ┌─ /src/parse/error.gleam:3:11
+  │
+3 │   let a = if wibble {
+  │           ^^ Gleam doesn't have if expressions
+
+If you want to write a conditional expression you can use a `case`:
+
+    case condition {
+      True -> todo
+      False -> todo
+    }
+
+See: https://tour.gleam.run/flow-control/case-expressions/

compiler-core/src/parse/tests.rs

@@ -1493,3 +1493,16 @@ type Wibble {
 "#
     );
 }
+
+#[test]
+fn if_like_expression() {
+    assert_module_error!(
+        r#"
+pub fn main() {
+  let a = if wibble {
+    wobble
+  }
+}
+"#
+    );
+}