feat: suggest lambda operator

crates/reporting/src/error/parse.rs

@@ -246,6 +246,16 @@ fn to_expr_report<'a>(
                     Context::InNode(Node::WhenBranch, _pos, _) => {
                         return to_unexpected_arrow_report(alloc, lines, filename, *pos, start);
                     }
+
+                    Context::InDef(_pos) => {
+                        vec![alloc.stack([
+                            alloc.concat([alloc.reflow(
+                                "Did you mean to define a function? Condider defining this as",
+                            )]),
+                            alloc.vcat(vec![alloc.text("id = \\x -> x")]),
+                        ])]
+                    }
+
                     _ => {
                         vec![alloc.stack([
                             alloc.concat([

crates/reporting/tests/test_reporting.rs

@@ -5653,11 +5653,9 @@ All branches in an `if` must have the same type!
     4│      main = 5 -> 3
                      ^^
 
-    The arrow -> is only used to define cases in a `when`.
+    Did you mean to define a function? Condider defining this as
 
-        when color is
+    id = \x -> x
-            Red -> "stop!"
-            Green -> "go!"
     "###
     );