Special-case some more list error messages

src/check/check_types.zig

@@ -332,6 +332,7 @@ pub fn checkExpr(self: *Self, expr_idx: CIR.Expr.Idx) void {
                                 .incompatible_elem_var = @enumFromInt(@intFromEnum(elem_expr_id)),
                                 .incompatible_elem_snapshot = incompatible_snapshot,
                                 .incompatible_elem_index = i,
+                                .list_length = elems.len,
                             },
                         });
 

src/check/check_types/problem.zig

@@ -222,38 +222,54 @@ pub const Problem = union(enum) {
 
         // Add description
         buf.clearRetainingCapacity();
-        try buf.appendSlice("The ");
-        try appendOrdinal(buf, data.first_elem_index + 1);
-        try buf.appendSlice(" and ");
-        try appendOrdinal(buf, data.incompatible_elem_index + 1);
-        try buf.appendSlice(" elements in this list have incompatible types:");
+        if (data.list_length == 2) {
+            // Special case for lists with exactly 2 elements
+            try buf.appendSlice("The two elements in this list have incompatible types:");
+        } else if (data.first_elem_index == 0 and data.incompatible_elem_index == 1) {
+            // Special case for first two elements in longer lists
+            try buf.appendSlice("The first two elements in this list have incompatible types:");
+        } else {
+            try buf.appendSlice("The ");
+            try appendOrdinal(buf, data.first_elem_index + 1);
+            try buf.appendSlice(" and ");
+            try appendOrdinal(buf, data.incompatible_elem_index + 1);
+            try buf.appendSlice(" elements in this list have incompatible types:");
+        }
         const owned_description = try report.addOwnedString(buf.items);
         try report.document.addText(owned_description);
         try report.document.addLineBreak();
 
-        // Show a single region spanning both mismatched elements
-        const span_start = if (data.first_elem_region.start.offset < data.incompatible_elem_region.start.offset)
-            data.first_elem_region.start
-        else
-            data.incompatible_elem_region.start;
-
-        const span_end = if (data.first_elem_region.end.offset > data.incompatible_elem_region.end.offset)
-            data.first_elem_region.end
-        else
-            data.incompatible_elem_region.end;
-
-        const span_region_info = base.RegionInfo.position(
+        // Show the two elements with separate underlines
+        // First element
+        const first_elem_region_info = base.RegionInfo.position(
             source,
             module_env.line_starts.items,
-            span_start.offset,
-            span_end.offset,
+            data.first_elem_region.start.offset,
+            data.first_elem_region.end.offset,
         ) catch return report;
         try report.document.addSourceRegion(
             source,
-            span_region_info.start_line_idx,
-            span_region_info.start_col_idx,
-            span_region_info.end_line_idx,
-            span_region_info.end_col_idx,
+            first_elem_region_info.start_line_idx,
+            first_elem_region_info.start_col_idx,
+            first_elem_region_info.end_line_idx,
+            first_elem_region_info.end_col_idx,
+            .error_highlight,
+            filename,
+        );
+
+        // Second element
+        const incompatible_elem_region_info = base.RegionInfo.position(
+            source,
+            module_env.line_starts.items,
+            data.incompatible_elem_region.start.offset,
+            data.incompatible_elem_region.end.offset,
+        ) catch return report;
+        try report.document.addSourceRegion(
+            source,
+            incompatible_elem_region_info.start_line_idx,
+            incompatible_elem_region_info.start_col_idx,
+            incompatible_elem_region_info.end_line_idx,
+            incompatible_elem_region_info.end_col_idx,
             .error_highlight,
             filename,
         );
@@ -454,6 +470,7 @@ pub const IncompatibleListElements = struct {
     incompatible_elem_var: Var,
     incompatible_elem_snapshot: SnapshotContentIdx,
     incompatible_elem_index: usize, // 0-based index of the incompatible element
+    list_length: usize, // Total number of elements in the list
 };
 
 /// A two var problem

src/snapshots/can_list_first_concrete.md

@@ -9,12 +9,17 @@ type=expr
 ~~~
 # PROBLEMS
 **INCOMPATIBLE LIST ELEMENTS**
-The first and second elements in this list have incompatible types:
-**can_list_first_concrete.md:1:2:1:13:**
+The first two elements in this list have incompatible types:
+**can_list_first_concrete.md:1:2:1:4:**
 ```roc
 [42, "world", 3.14]
 ```
- ^^^^^^^^^^^
+ ^^
+**can_list_first_concrete.md:1:6:1:13:**
+```roc
+[42, "world", 3.14]
+```
+     ^^^^^^^
 
 The first element has this type:
     _Num(*)_

src/snapshots/can_list_heterogeneous.md

@@ -9,12 +9,17 @@ type=expr
 ~~~
 # PROBLEMS
 **INCOMPATIBLE LIST ELEMENTS**
-The first and second elements in this list have incompatible types:
-**can_list_heterogeneous.md:1:2:1:12:**
+The first two elements in this list have incompatible types:
+**can_list_heterogeneous.md:1:2:1:3:**
 ```roc
 [1, "hello", 3.14]
 ```
- ^^^^^^^^^^
+ ^
+**can_list_heterogeneous.md:1:5:1:12:**
+```roc
+[1, "hello", 3.14]
+```
+    ^^^^^^^
 
 The first element has this type:
     _Num(*)_

src/snapshots/can_list_multiline_mismatch.md

@@ -0,0 +1,72 @@
+# META
+~~~ini
+description=Multiline list with type mismatch
+type=expr
+~~~
+# SOURCE
+~~~roc
+[
+    42,
+    "hello world",
+    100
+]
+~~~
+# PROBLEMS
+**INCOMPATIBLE LIST ELEMENTS**
+The first two elements in this list have incompatible types:
+**can_list_multiline_mismatch.md:2:5:2:7:**
+```roc
+    42,
+```
+    ^^
+**can_list_multiline_mismatch.md:3:5:3:18:**
+```roc
+    "hello world",
+```
+    ^^^^^^^^^^^^^
+
+The first element has this type:
+    _Num(*)_
+
+However, the second element has this type:
+    _Str_
+
+All elements in a list must have compatible types.
+
+# TOKENS
+~~~zig
+OpenSquare(1:1-1:2),Newline(1:1-1:1),
+Int(2:5-2:7),Comma(2:7-2:8),Newline(1:1-1:1),
+StringStart(3:5-3:6),StringPart(3:6-3:17),StringEnd(3:17-3:18),Comma(3:18-3:19),Newline(1:1-1:1),
+Int(4:5-4:8),Newline(1:1-1:1),
+CloseSquare(5:1-5:2),EndOfFile(5:2-5:2),
+~~~
+# PARSE
+~~~clojure
+(e-list @1-1-5-2
+	(e-int @2-5-2-7 (raw "42"))
+	(e-string @3-5-3-18
+		(e-string-part @3-6-3-17 (raw "hello world")))
+	(e-int @4-5-4-8 (raw "100")))
+~~~
+# FORMATTED
+~~~roc
+[
+	42,
+	"hello world",
+	100,
+]
+~~~
+# CANONICALIZE
+~~~clojure
+(e-list @1-1-5-2 (elem-var 72) (id 76)
+	(elems
+		(e-int @2-5-2-7 (value "42"))
+		(e-string @3-5-3-18
+			(e-literal @3-6-3-17 (string "hello world")))
+		(e-int @4-5-4-8 (value "100"))))
+~~~
+# TYPES
+~~~clojure
+(expr (id 76) (type "List(Error)"))
+~~~

src/snapshots/can_list_nested_heterogeneous.md

@@ -10,11 +10,16 @@ type=expr
 # PROBLEMS
 **INCOMPATIBLE LIST ELEMENTS**
 The second and third elements in this list have incompatible types:
-**can_list_nested_heterogeneous.md:1:6:1:20:**
+**can_list_nested_heterogeneous.md:1:6:1:9:**
 ```roc
 [[], [1], ["hello"]]
 ```
-     ^^^^^^^^^^^^^^
+     ^^^
+**can_list_nested_heterogeneous.md:1:11:1:20:**
+```roc
+[[], [1], ["hello"]]
+```
+          ^^^^^^^^^
 
 The second element has this type:
     _List(Num(*))_

src/snapshots/can_list_number_doesnt_fit.md

@@ -15,12 +15,17 @@ This number literal is not valid: 1u8
 This number literal is not valid: 2u8
 
 **INCOMPATIBLE LIST ELEMENTS**
-The first and second elements in this list have incompatible types:
-**can_list_number_doesnt_fit.md:1:2:1:10:**
+The first two elements in this list have incompatible types:
+**can_list_number_doesnt_fit.md:1:2:1:5:**
 ```roc
 [1u8, 2u8, 300]
 ```
- ^^^^^^^^
+ ^^^
+**can_list_number_doesnt_fit.md:1:7:1:10:**
+```roc
+[1u8, 2u8, 300]
+```
+      ^^^
 
 The first element has this type:
     _Error_

src/snapshots/can_list_triple_nested_heterogeneous.md

@@ -10,11 +10,16 @@ type=expr
 # PROBLEMS
 **INCOMPATIBLE LIST ELEMENTS**
 The second and third elements in this list have incompatible types:
-**can_list_triple_nested_heterogeneous.md:1:6:1:32:**
+**can_list_triple_nested_heterogeneous.md:1:6:1:15:**
 ```roc
 [[], [[], [1]], [[], ["hello"]]]
 ```
-     ^^^^^^^^^^^^^^^^^^^^^^^^^^
+     ^^^^^^^^^
+**can_list_triple_nested_heterogeneous.md:1:17:1:32:**
+```roc
+[[], [[], [1]], [[], ["hello"]]]
+```
+                ^^^^^^^^^^^^^^^
 
 The second element has this type:
     _List(List(Num(*)))_

src/snapshots/can_list_two_elements.md

@@ -0,0 +1,58 @@
+# META
+~~~ini
+description=List with exactly two incompatible elements
+type=expr
+~~~
+# SOURCE
+~~~roc
+[1, "hello"]
+~~~
+# PROBLEMS
+**INCOMPATIBLE LIST ELEMENTS**
+The two elements in this list have incompatible types:
+**can_list_two_elements.md:1:2:1:3:**
+```roc
+[1, "hello"]
+```
+ ^
+**can_list_two_elements.md:1:5:1:12:**
+```roc
+[1, "hello"]
+```
+    ^^^^^^^
+
+The first element has this type:
+    _Num(*)_
+
+However, the second element has this type:
+    _Str_
+
+All elements in a list must have compatible types.
+
+# TOKENS
+~~~zig
+OpenSquare(1:1-1:2),Int(1:2-1:3),Comma(1:3-1:4),StringStart(1:5-1:6),StringPart(1:6-1:11),StringEnd(1:11-1:12),CloseSquare(1:12-1:13),EndOfFile(1:13-1:13),
+~~~
+# PARSE
+~~~clojure
+(e-list @1-1-1-13
+	(e-int @1-2-1-3 (raw "1"))
+	(e-string @1-5-1-12
+		(e-string-part @1-6-1-11 (raw "hello"))))
+~~~
+# FORMATTED
+~~~roc
+NO CHANGE
+~~~
+# CANONICALIZE
+~~~clojure
+(e-list @1-1-1-13 (elem-var 72) (id 75)
+	(elems
+		(e-int @1-2-1-3 (value "1"))
+		(e-string @1-5-1-12
+			(e-literal @1-6-1-11 (string "hello")))))
+~~~
+# TYPES
+~~~clojure
+(expr (id 75) (type "List(Error)"))
+~~~

src/snapshots/can_nested_heterogeneous_lists.md

@@ -9,12 +9,17 @@ type=expr
 ~~~
 # PROBLEMS
 **INCOMPATIBLE LIST ELEMENTS**
-The first and second elements in this list have incompatible types:
-**can_nested_heterogeneous_lists.md:1:3:1:13:**
+The two elements in this list have incompatible types:
+**can_nested_heterogeneous_lists.md:1:3:1:4:**
 ```roc
 [[1, "hello"], [2, 3]]
 ```
-  ^^^^^^^^^^
+  ^
+**can_nested_heterogeneous_lists.md:1:6:1:13:**
+```roc
+[[1, "hello"], [2, 3]]
+```
+     ^^^^^^^
 
 The first element has this type:
     _Num(*)_
@@ -25,12 +30,17 @@ However, the second element has this type:
 All elements in a list must have compatible types.
 
 **INCOMPATIBLE LIST ELEMENTS**
-The first and second elements in this list have incompatible types:
-**can_nested_heterogeneous_lists.md:1:2:1:22:**
+The two elements in this list have incompatible types:
+**can_nested_heterogeneous_lists.md:1:2:1:14:**
 ```roc
 [[1, "hello"], [2, 3]]
 ```
- ^^^^^^^^^^^^^^^^^^^^
+ ^^^^^^^^^^^^
+**can_nested_heterogeneous_lists.md:1:16:1:22:**
+```roc
+[[1, "hello"], [2, 3]]
+```
+               ^^^^^^
 
 The first element has this type:
     _List(Error)_

src/snapshots/expr/list_type_err.md

@@ -10,11 +10,16 @@ type=expr
 # PROBLEMS
 **INCOMPATIBLE LIST ELEMENTS**
 The second and third elements in this list have incompatible types:
-**list_type_err.md:1:5:1:15:**
+**list_type_err.md:1:5:1:6:**
 ```roc
 [1, 2, "hello"]
 ```
-    ^^^^^^^^^^
+    ^
+**list_type_err.md:1:8:1:15:**
+```roc
+[1, 2, "hello"]
+```
+       ^^^^^^^
 
 The second element has this type:
     _Num(*)_

test_list_messages.txt

@@ -0,0 +1,11 @@
+List with 2 elements, both incompatible:
+[1, "hello"]
+→ "The two elements in this list have incompatible types"
+
+List with 3+ elements, first two incompatible:
+[1, "hello", 3.14]
+→ "The first two elements in this list have incompatible types"
+
+List with 3+ elements, later elements incompatible:
+[1, 2, "hello"]
+→ "The second and third elements in this list have incompatible types"