elem_in_list

compiler/constrain/src/expr.rs

@@ -202,9 +202,12 @@ pub fn constrain_expr(
                 let list_elem_type = Type::Variable(*elem_var);
                 let mut constraints = Vec::with_capacity(1 + loc_elems.len());
 
-                for loc_elem in loc_elems {
+                for (index, loc_elem) in loc_elems.iter().enumerate() {
-                    let elem_expected =
+                    let elem_expected = ForReason(
-                        ForReason(Reason::ElemInList, list_elem_type.clone(), region);
+                        Reason::ElemInList { index },
+                        list_elem_type.clone(),
+                        loc_elem.region,
+                    );
                     let constraint =
                         constrain_expr(env, loc_elem.region, &loc_elem.value, elem_expected);
 

compiler/constrain/src/uniq.rs

@@ -596,9 +596,12 @@ pub fn constrain_expr(
                 let entry_type = Type::Variable(*elem_var);
                 let mut constraints = Vec::with_capacity(1 + loc_elems.len());
 
-                for loc_elem in loc_elems.iter() {
+                for (index, loc_elem) in loc_elems.iter().enumerate() {
-                    let elem_expected =
+                    let elem_expected = Expected::ForReason(
-                        Expected::ForReason(Reason::ElemInList, entry_type.clone(), region);
+                        Reason::ElemInList { index },
+                        entry_type.clone(),
+                        region,
+                    );
                     let constraint = constrain_expr(
                         env,
                         var_store,

compiler/reporting/src/type_error.rs

@@ -277,6 +277,27 @@ fn to_expr_report(
                     ]),
                 )
             }
+            Reason::ElemInList { index } => {
+                // NOTE: is 0-based
+
+                let ith = int_to_ordinal(index + 1);
+
+                report_mismatch(
+                    filename,
+                    &category,
+                    found,
+                    expected_type,
+                    region,
+                    Some(expr_region),
+                    plain_text(&format!(
+                        "The {} element of this list does not match all the previous elements",
+                        ith
+                    )),
+                    plain_text(&format!("The {} element is ", ith)),
+                    plain_text("but all the previous elements in the list have type"),
+                    plain_text("instead. I need all elements of a list to have the same type!"),
+                )
+            }
             other => {
                 //    AnonymousFnArg { arg_index: u8 },
                 //    NamedFnArg(String /* function name */, u8 /* arg index */),
@@ -288,7 +309,6 @@ fn to_expr_report(
                 //    IntLiteral,
                 //    NumLiteral,
                 //    InterpolatedStringVar,
-                //    ElemInList,
                 //    RecordUpdateValue(Lowercase),
                 //    RecordUpdateKeys(Symbol, SendMap<Lowercase, Type>),
                 todo!("I don't have a message yet for reason {:?}", other)

compiler/reporting/tests/test_reporting.rs

@@ -899,6 +899,35 @@ mod test_reporting {
         )
     }
 
+    #[test]
+    fn elem_in_list() {
+        report_problem_as(
+            indoc!(
+                r#"
+                [ 1, 3, "foo" ]
+                "#
+            ),
+            indoc!(
+                r#"
+                The 3rd element of this list does not match all the previous elements
+
+                1 ┆  [ 1, 3, "foo" ]
+                  ┆          ^^^^^
+
+                The 3rd element is a string of type
+
+                    Str
+
+                but all the previous elements in the list have type
+
+                    Num a
+
+                instead. I need all elements of a list to have the same type!
+                "#
+            ),
+        )
+    }
+
     #[test]
     fn circular_type() {
         report_problem_as(

compiler/types/src/types.rs

@@ -628,7 +628,7 @@ pub enum Reason {
     WhenGuard,
     IfCondition,
     IfBranch { index: usize, total_branches: usize },
-    ElemInList,
+    ElemInList { index: usize },
     RecordUpdateValue(Lowercase),
     RecordUpdateKeys(Symbol, SendMap<Lowercase, Type>),
 }