From b3f3f08c9631f5a124bc736178b15c5785aced3c Mon Sep 17 00:00:00 2001
From: Richard Feldman <richard.t.feldman@gmail.com>
Date: Mon, 20 May 2019 22:02:24 -0400
Subject: [PATCH] Add Expr::Let

---
 src/expr.rs  |  1 +
 src/parse.rs | 20 ++++++++++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/src/expr.rs b/src/expr.rs
index d2dbe0a6eb..d2f0daff28 100644
--- a/src/expr.rs
+++ b/src/expr.rs
@@ -8,6 +8,7 @@ pub enum Expr {
     Char(char),
 
     Var(String),
+    Let(String, Box<Expr>, Box<Expr>),
 
     // Functions
     Func(String, Box<Expr>),
diff --git a/src/parse.rs b/src/parse.rs
index 34e2f04a23..c24f44e5ac 100644
--- a/src/parse.rs
+++ b/src/parse.rs
@@ -66,6 +66,7 @@ parser! {
             number_literal(),
             char_literal(),
             if_expr(),
+            let_expr(),
             func_or_var(),
         ))
         .and(
@@ -155,6 +156,25 @@ where I: Stream<Item = char, Position = IndentablePosition>,
     ))
 }
 
+pub fn let_expr<I>() -> impl Parser<Input = I, Output = Expr>
+where I: Stream<Item = char, Position = IndentablePosition>,
+    I::Error: ParseError<I::Item, I::Range, I::Position>
+{
+    ident()
+        .skip(spaces())
+        .skip(char('='))
+        .skip(spaces())
+        .and(expr_body())
+        // TODO check for indent in this expr, then parse a second expr post-outdent
+        .and(expr_body())
+        .map(|((var_name, var_expr), in_expr)| {
+            match var_name {
+                Ok(str) => Expr::Let(str, Box::new(var_expr), Box::new(in_expr)),
+                Err(_ident_problem) => Expr::SyntaxProblem("TODO put _ident_problem here".to_owned())
+            }
+        })
+}
+
 pub fn func_or_var<I>() -> impl Parser<Input = I, Output = Expr>
 where I: Stream<Item = char, Position = IndentablePosition>,
     I::Error: ParseError<I::Item, I::Range, I::Position>