Implement parser & formatter for Pair annotations.

crates/aiken-lang/src/format.rs

@@ -424,7 +424,14 @@ impl<'comments> Formatter<'comments> {
             Annotation::Tuple { elems, .. } => {
                 wrap_args(elems.iter().map(|t| (self.annotation(t), false)))
             }
-            Annotation::Pair { .. } => todo!(),
+            Annotation::Pair { fst, snd, .. } => "Pair"
+                .to_doc()
+                .append("<")
+                .append(self.annotation(fst))
+                .append(break_(",", ", "))
+                .append(self.annotation(snd))
+                .append(">")
+                .group(),
         }
         .group()
     }

crates/aiken-lang/src/parser/annotation.rs

@@ -1,8 +1,9 @@
-use chumsky::prelude::*;
-
-use crate::ast;
-
 use super::{error::ParseError, token::Token};
+use crate::{
+    ast,
+    builtins::{PAIR, PRELUDE},
+};
+use chumsky::prelude::*;
 
 pub fn parser() -> impl Parser<Token, ast::Annotation, Error = ParseError> {
     recursive(|expression| {
@@ -14,6 +15,31 @@ pub fn parser() -> impl Parser<Token, ast::Annotation, Error = ParseError> {
                     name,
                 }
             }),
+            // Pair
+            select! {Token::Name { name } if name == PRELUDE => name}
+                .then_ignore(just(Token::Dot))
+                .or_not()
+                .then_ignore(select! {Token::UpName { name } if name == PAIR => name})
+                .ignore_then(
+                    expression
+                        .clone()
+                        .separated_by(just(Token::Comma))
+                        .exactly(2)
+                        .delimited_by(just(Token::Less), just(Token::Greater)),
+                )
+                .map_with_span(|elems: Vec<ast::Annotation>, span| ast::Annotation::Pair {
+                    location: span,
+                    fst: elems
+                        .first()
+                        .expect("Pair should have exactly 2 elements")
+                        .to_owned()
+                        .into(),
+                    snd: elems
+                        .last()
+                        .expect("Pair should have exactly 2 elements")
+                        .to_owned()
+                        .into(),
+                }),
             // Tuple
             expression
                 .clone()

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

@@ -792,3 +792,13 @@ fn superfluous_parens_in_binop() {
         "#
     );
 }
+
+#[test]
+fn format_pairs() {
+    assert_format!(
+        r#"
+        pub fn foo(x: Pair<Int, Int>) {
+            Pair(x.1st, x.2nd)
+        }"#
+    );
+}

crates/aiken-lang/src/tests/snapshots/format_pairs.snap

@@ -0,0 +1,7 @@
+---
+source: crates/aiken-lang/src/tests/format.rs
+description: "Code:\n\npub fn foo(x: Pair<Int, Int>) {\n    Pair(x.1st, x.2nd)\n}"
+---
+pub fn foo(x: Pair<Int, Int>) {
+  Pair(x.1st, x.2nd)
+}