Fixes #767

Co-authored-by: @rvcas <x@rvcas.dev>

CHANGELOG.md

@@ -42,6 +42,7 @@
 - **aiken-lang**: function aliases now resolved to the module and function name in codegen. @Microproofs
 - **aiken-lang**: fix indentation of pipelines to remain a multiple of the base indent increment. @KtorZ
 - **aiken-lang**: forbid presence of non-serialisable data-types in compound structures like List and Tuple. @KtorZ
+- **aiken-lang**: fix 'given' arity reported by 'incorrect arity' error message. @rvcas
 
 ### Changed
 

crates/aiken-lang/src/tests/check.rs

@@ -1595,6 +1595,57 @@ fn pipe_with_wrong_type_and_full_args() {
     ))
 }
 
+#[test]
+fn pipe_wrong_arity_partially_applied() {
+    let source_code = r#"
+        fn f(_a: Int, _b: Int, _c: Int) -> Int {
+            todo
+        }
+
+        test foo() {
+            0 |> f(0)
+        }
+    "#;
+
+    assert!(matches!(
+        check(parse(source_code)),
+        Err((_, Error::IncorrectFieldsArity { given, expected, .. })) if given == 2 && expected == 3
+    ))
+}
+
+#[test]
+fn pipe_wrong_arity_fully_saturated() {
+    let source_code = r#"
+        fn f(_a: Int, _b: Int, _c: Int) -> Int {
+            todo
+        }
+
+        test foo() {
+            0 |> f(0, 0, 0)
+        }
+    "#;
+
+    assert!(matches!(
+        check(parse(source_code)),
+        Err((_, Error::NotFn { .. }))
+    ))
+}
+
+#[test]
+fn pipe_wrong_arity_fully_saturated_return_fn() {
+    let source_code = r#"
+        fn f(_a: Int, _b: Int, _c: Int) -> fn(Int) -> Int {
+            todo
+        }
+
+        test foo() {
+            (0 |> f(0, 0, 0)) == 0
+        }
+    "#;
+
+    assert!(check(parse(source_code)).is_ok());
+}
+
 #[test]
 fn fuzzer_ok_basic() {
     let source_code = r#"

crates/aiken-lang/src/tipo/pipe.rs

@@ -92,7 +92,7 @@ impl<'a, 'b, 'c> PipeTyper<'a, 'b, 'c> {
 
                     match fun.tipo().fn_arity() {
                         // Rewrite as right(left, ..args)
-                        Some(arity) if arity == arguments.len() + 1 => {
+                        Some(arity) if arguments.len() < arity => {
                             self.infer_insert_pipe(fun, arguments, location)?
                         }