From 313c044b4a878a425aaca6554576f5154ace8ff9 Mon Sep 17 00:00:00 2001
From: Joel Kronqvist <joel.kronqvist@iki.fi>
Date: Wed, 6 Aug 2025 16:23:18 +0300
Subject: Implemented let-bindings

---
 src/type/check.rs   | 39 +++++++++++++++++++--------------------
 src/type/display.rs |  7 +++++++
 src/type/mod.rs     |  2 ++
 3 files changed, 28 insertions(+), 20 deletions(-)

(limited to 'src/type')

diff --git a/src/type/check.rs b/src/type/check.rs
index d0ce807..e536ce7 100644
--- a/src/type/check.rs
+++ b/src/type/check.rs
@@ -1,6 +1,6 @@
 
 use crate::r#type::{Type, TypeError, Type::*, TypeError::*, util::*};
-use crate::sexp::{SExp, SExp::*, SLeaf::*};
+use crate::sexp::{SExp, SExp::*, SLeaf::*, util::*};
 use std::collections::HashMap;
 
 impl SExp {
@@ -101,23 +101,6 @@ impl SExp {
     /// );
     /// ```
     ///
-    /// As such it must be that the type of the let-binding s-expression is st.
-    /// ```rust
-    /// use myslip::{
-    ///     r#type::{*, Type::*, TypeError::*, util::*},
-    ///     sexp::{SExp::*, SLeaf::*, util::*},
-    /// };
-    ///
-    /// assert_eq!(
-    ///     scons(Let, scons(var("x"), scons(1, Nil))).type_check(),
-    ///     Ok(arr(vt("T"), vt("T")))
-    /// );
-    /// ```
-    /// ie. the identity function.
-    ///
-    /// No more behavior on typing let can be defined, as typing the let-keyword
-    /// would require the yet untyped variable to be typed.
-    ///
     /// Though perhaps the most important task of the type system
     /// is to increase safety by being able to warn about errors
     /// before evaluation. Here are some failing examples:
@@ -184,7 +167,7 @@ impl SExp {
     }
 
 
-    fn infer_type(&self, ctx: HashMap<String, Type>) -> Result<Type, TypeError> {
+    fn infer_type(&self, mut ctx: HashMap<String, Type>) -> Result<Type, TypeError> {
 
         match self {
 
@@ -210,13 +193,29 @@ impl SExp {
                                 vecof(vt("T")),
                                 List(vec![VecType, vecof(vt("T"))])
                             )),
-            Atom(Let)    => todo!(),
+            Atom(Let)    => Ok(LetType),
 
 
             SCons(op, l) => {
+
+                // Let-expressions
+                if let Some((varname, val)) = (*op).clone().check_let() {
+                    let valtype = val.infer_type(ctx.clone())?;
+                    ctx.insert(varname, valtype);
+                    return match (**l).clone() {
+                        SCons(exp, nil) if *nil == Atom(Nil) => *exp,
+                        t => t
+                    }.infer_type(ctx);
+                }
+                if **op == Atom(Let) {
+                    return Err(LetAsOperator(scons(op.clone(), l.clone())));
+                }
+
+                // Normal operation
                 let opertype = (*op).infer_type(ctx.clone())?;
                 let argstype =  (*l).infer_list_type(ctx)?;
 
+
                 let argstype = if opertype == arr(
                     vecof(vt("T")),
                     List(vec![VecType, vecof(vt("T"))])
diff --git a/src/type/display.rs b/src/type/display.rs
index fe54941..4a9b54a 100644
--- a/src/type/display.rs
+++ b/src/type/display.rs
@@ -103,6 +103,13 @@ impl fmt::Display for TypeError {
                     argtype, generictype
                 )
             },
+            LetAsOperator(letexp) => {
+                write!(
+                    f,
+                    "let used as operator instead of generating an operator: maybe try '({} ...)' instead",
+                    letexp
+                )
+            },
             OtherError => write!(f, "uncategorized error"),
         }
     }
diff --git a/src/type/mod.rs b/src/type/mod.rs
index 9e0d9ec..afb5a0b 100644
--- a/src/type/mod.rs
+++ b/src/type/mod.rs
@@ -59,6 +59,8 @@ pub enum TypeError {
         generictype: Type,
     },
 
+    LetAsOperator(SExp),
+
     OtherError
 }
 
-- 
cgit v1.2.3