Experiment with monadic bind.

crates/aiken-lang/src/parser/expr/assignment.rs

@@ -1,10 +1,9 @@
-use chumsky::prelude::*;
-
 use crate::{
     ast,
     expr::UntypedExpr,
     parser::{annotation, error::ParseError, pattern, token::Token},
 };
+use chumsky::prelude::*;
 
 pub fn let_(
     r: Recursive<'_, Token, UntypedExpr, ParseError>,
@@ -12,9 +11,9 @@ pub fn let_(
     just(Token::Let)
         .ignore_then(pattern())
         .then(just(Token::Colon).ignore_then(annotation()).or_not())
-        .then_ignore(just(Token::Equal))
+        .then(choice((just(Token::Equal), just(Token::LArrow))))
         .then(r.clone())
-        .validate(move |((pattern, annotation), value), span, emit| {
+        .validate(move |(((pattern, annotation), kind), value), span, emit| {
             if matches!(value, UntypedExpr::Assignment { .. }) {
                 emit(ParseError::invalid_assignment_right_hand_side(span))
             }
@@ -23,7 +22,11 @@ pub fn let_(
                 location: span,
                 value: Box::new(value),
                 pattern,
-                kind: ast::AssignmentKind::Let,
+                kind: if kind == Token::LArrow {
+                    ast::AssignmentKind::Bind
+                } else {
+                    ast::AssignmentKind::Let
+                },
                 annotation,
             }
         })

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

@@ -163,6 +163,8 @@ pub fn lexer() -> impl Parser<char, Vec<(Token, Span)>, Error = ParseError> {
         just("!=").to(Token::NotEqual),
         just('!').to(Token::Bang),
         just('?').to(Token::Question),
+        just("<-").to(Token::LArrow),
+        just("->").to(Token::RArrow),
         choice((
             just("<=").to(Token::LessEqual),
             just('<').to(Token::Less),
@@ -170,7 +172,6 @@ pub fn lexer() -> impl Parser<char, Vec<(Token, Span)>, Error = ParseError> {
             just('>').to(Token::Greater),
         )),
         just('+').to(Token::Plus),
-        just("->").to(Token::RArrow),
         just('-').to(Token::Minus),
         just('*').to(Token::Star),
         just('/').to(Token::Slash),

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

@@ -62,6 +62,7 @@ pub enum Token {
     Pipe,        // '|>'
     Dot,         // '.'
     RArrow,      // '->'
+    LArrow,      // '<-'
     DotDot,      // '..'
     EndOfFile,
     // Docs/Extra
@@ -152,6 +153,7 @@ impl fmt::Display for Token {
             Token::Pipe => "|>",
             Token::Dot => ".",
             Token::RArrow => "->",
+            Token::LArrow => "<-",
             Token::DotDot => "..",
             Token::EndOfFile => "EOF",
             Token::Comment => "//",