Implement 'let'

2 files changed, 10 insertions, 0 deletions

diff --git a/AST.hs b/AST.hs
index eae5af8..df946dd 100644
--- a/AST.hs
+++ b/AST.hs
@@ -14,6 +14,7 @@ data Value
     | VName Name
     | VQuoted Value
     | VLambda [Name] Value
+    | VLet [(Name, Value)] Value
     | VBuiltin String
     | VEllipsis
   deriving (Eq)
@@ -29,6 +30,7 @@ instance Show Value where
     show (VName n) = n
     show (VQuoted e) = '\'' : show e
     show (VLambda as v) = "(lambda (" ++ intercalate " " as ++ ") " ++ show v ++ ")"
+    show (VLet ps v) = "(let (" ++ intercalate " " ["(" ++ n ++ " " ++ show w ++ ")" | (n, w) <- ps] ++ ") " ++ show v ++ ")"
     show (VBuiltin str) = "[[builtin " ++ str ++ "]]"
     show VEllipsis = "..."
 
diff --git a/Compiler.hs b/Compiler.hs
index 9f8b595..a5a1f3d 100644
--- a/Compiler.hs
+++ b/Compiler.hs
@@ -36,6 +36,13 @@ analyseValue = go []
         | Just names <- mapM fromVName args = go scopes (VLambda names body)
         | otherwise = error "Invalid 'lambda' syntax: Invalid argument list"
     go _ (VList (VName "lambda" : _)) = error "Invalid 'lambda' syntax"
+    go scopes (VList [VName "let", VList args, body]) =
+        case sequence [if length p == 2 then Just (p !! 0, p !! 1) else Nothing | VList p <- args] of
+            Just pairs ->
+                go scopes (VList (VList [VName "lambda", VList (map fst pairs), body] : map snd pairs))
+            Nothing ->
+                error "Invalid 'let' syntax: Invalid variable list (not all pairs)"
+    go _ (VList (VName "let" : _)) = error "Invalid 'let' syntax: Invalid argument list"
     go scopes (VList values) = TVList (map (go scopes) values)
     go _ (VNum n) = TVNum n
     go _ (VString s) = TVString s
@@ -46,6 +53,7 @@ analyseValue = go []
         in TVLambda args t (Set.toList (collectEscapes 0 t))
     go _ (VBuiltin _) = undefined
     go _ VEllipsis = TVEllipsis
+    go _ (VLet _ _) = error "'let' should not be present yet at this point"
 
     collectEscapes :: Int -> TaggedValue -> Set.Set Name
     collectEscapes limit (TVList values) = Set.unions (map (collectEscapes limit) values)