Fix ordinal index on pairs

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

@@ -2349,3 +2349,52 @@ fn partial_eq_callback_return() {
         Err((_, Error::CouldNotUnify { .. }))
     ));
 }
+
+#[test]
+fn pair_access_on_call() {
+    let source_code = r#"
+        use aiken/builtin
+
+        pub fn list_at(xs: List<a>, index: Int) -> a {
+          if index == 0 {
+            builtin.head_list(xs)
+          } else {
+            list_at(builtin.tail_list(xs), index - 1)
+          }
+        }
+
+        fn foo() {
+          [list_at([Pair(1, 2)], 0).2nd, ..[1, 2]]
+        }
+    "#;
+
+    assert!(check(parse(source_code)).is_ok())
+}
+
+#[test]
+fn pair_index_out_of_bound() {
+    let source_code = r#"
+        pub fn foo() {
+            Pair(1, 2).3rd
+        }
+    "#;
+
+    assert!(matches!(
+        dbg!(check_validator(parse(source_code))),
+        Err((_, Error::PairIndexOutOfBound { .. }))
+    ))
+}
+
+#[test]
+fn not_indexable() {
+    let source_code = r#"
+        pub fn foo() {
+            "foo".1st
+        }
+    "#;
+
+    assert!(matches!(
+        dbg!(check_validator(parse(source_code))),
+        Err((_, Error::NotIndexable { .. }))
+    ))
+}

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

@@ -508,18 +508,16 @@ If you really meant to return that last expression, try to replace it with the f
         name: String,
     },
 
-    #[error(
+    #[error("I tripped over an attempt to access elements on something that isn't indexable.\n")]
-        "I tripped over an attempt to access tuple elements on something else than a tuple.\n"
-    )]
     #[diagnostic(url("https://aiken-lang.org/language-tour/primitive-types#tuples"))]
-    #[diagnostic(code("illegal::tuple_index"))]
+    #[diagnostic(code("illegal::indexable"))]
     #[diagnostic(help(
-        r#"Because you used a tuple-index on an element, I assumed it had to be a tuple but instead I found something of type:
+        r#"Because you used an ordinal index on an element, I assumed it had to be a tuple or a pair but instead I found something of type:
 
 ╰─▶ {type_info}"#,
         type_info = tipo.to_pretty(0).if_supports_color(Stdout, |s| s.red())
     ))]
-    NotATuple {
+    NotIndexable {
         #[label]
         location: Span,
         tipo: Rc<Type>,
@@ -675,12 +673,25 @@ You can help me by providing a type-annotation for 'x', as such:
     #[diagnostic(url("https://aiken-lang.org/language-tour/primitive-types#tuples"))]
     #[diagnostic(code("invalid::tuple_index"))]
     TupleIndexOutOfBound {
-        #[label]
+        #[label("out of bounds")]
         location: Span,
         index: usize,
         size: usize,
     },
 
+    #[error(
+        "I discovered an attempt to access the {} element of a {}.\n",
+        Ordinal(*index + 1).to_string().if_supports_color(Stdout, |s| s.purple()),
+        "Pair".if_supports_color(Stdout, |s| s.bright_blue()).if_supports_color(Stdout, |s| s.bold()),
+    )]
+    #[diagnostic(url("https://aiken-lang.org/language-tour/primitive-types#pairs"))]
+    #[diagnostic(code("invalid::pair_index"))]
+    PairIndexOutOfBound {
+        #[label("out of bounds")]
+        location: Span,
+        index: usize,
+    },
+
     #[error(
         "I tripped over the following labeled argument: {}.\n",
         label.if_supports_color(Stdout, |s| s.purple())
@@ -1035,7 +1046,7 @@ impl ExtraData for Error {
             | Error::MissingVarInAlternativePattern { .. }
             | Error::MultiValidatorEqualArgs { .. }
             | Error::NonLocalClauseGuardVariable { .. }
-            | Error::NotATuple { .. }
+            | Error::NotIndexable { .. }
             | Error::NotExhaustivePatternMatch { .. }
             | Error::NotFn { .. }
             | Error::PositionalArgumentAfterLabeled { .. }
@@ -1045,6 +1056,7 @@ impl ExtraData for Error {
             | Error::RecursiveType { .. }
             | Error::RedundantMatchClause { .. }
             | Error::TupleIndexOutOfBound { .. }
+            | Error::PairIndexOutOfBound { .. }
             | Error::UnexpectedLabeledArg { .. }
             | Error::UnexpectedLabeledArgInPattern { .. }
             | Error::UnknownLabels { .. }

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

@@ -2065,13 +2065,13 @@ impl<'a, 'b> ExprTyper<'a, 'b> {
 
     fn infer_tuple_index(
         &mut self,
-        tuple: UntypedExpr,
+        tuple_or_pair: UntypedExpr,
         index: usize,
         location: Span,
     ) -> Result<TypedExpr, Error> {
-        let tuple = self.infer(tuple)?;
+        let tuple_or_pair = self.infer(tuple_or_pair)?;
 
-        let tipo = match *collapse_links(tuple.tipo()) {
+        let tipo = match *collapse_links(tuple_or_pair.tipo()) {
             Type::Tuple {
                 ref elems,
                 alias: _,
@@ -2087,9 +2087,22 @@ impl<'a, 'b> ExprTyper<'a, 'b> {
                     Ok(elems[index].clone())
                 }
             }
-            _ => Err(Error::NotATuple {
+            Type::Pair {
+                ref fst,
+                ref snd,
+                alias: _,
+            } => {
+                if index == 0 {
+                    Ok(fst.clone())
+                } else if index == 1 {
+                    Ok(snd.clone())
+                } else {
+                    Err(Error::PairIndexOutOfBound { location, index })
+                }
+            }
+            _ => Err(Error::NotIndexable {
                 location,
-                tipo: tuple.tipo(),
+                tipo: tuple_or_pair.tipo(),
             }),
         }?;
 
@@ -2097,7 +2110,7 @@ impl<'a, 'b> ExprTyper<'a, 'b> {
             location,
             tipo,
             index,
-            tuple: Box::new(tuple),
+            tuple: Box::new(tuple_or_pair),
         })
     }
 

crates/aiken-project/src/tests/gen_uplc.rs

@@ -2159,7 +2159,7 @@ fn acceptance_test_23_to_list() {
         }
 
         test to_list_2() {
-            to_list(fixture_1()) == [Pair("foo", 42).2nd, Pair("bar", 14)]
+            to_list(fixture_1()) == [Pair("foo", 42), Pair("bar", 14)]
         }
     "#;