3 files changed, 35 insertions, 8 deletions
diff --git a/lambda-calcul/haskell/src/Minilang/Lambda/Eval.hs b/lambda-calcul/haskell/src/Minilang/Lambda/Eval.hs
index a15d3f9..cbbdc51 100644
--- a/lambda-calcul/haskell/src/Minilang/Lambda/Eval.hs
+++ b/lambda-calcul/haskell/src/Minilang/Lambda/Eval.hs
@@ -13,7 +13,7 @@ type Env = [(Text, Value)]
 
 data Value
   = V Text
-  | Abs Text Term Env
+  | Abs Text Term Env -- a closure, ie. lambda with its captured environment
   | Ap Value Value
   deriving (Show, Read, Eq)
 
diff --git a/lambda-calcul/haskell/src/Minilang/Lambda/Parser.hs b/lambda-calcul/haskell/src/Minilang/Lambda/Parser.hs
index 37834d3..426a8e8 100644
--- a/lambda-calcul/haskell/src/Minilang/Lambda/Parser.hs
+++ b/lambda-calcul/haskell/src/Minilang/Lambda/Parser.hs
@@ -4,15 +4,16 @@ module Minilang.Lambda.Parser where
 
 import Control.Applicative (Alternative (many), (<|>))
 import Data.Bifunctor (first)
+import Data.Functor (void)
 import qualified Data.List as List
-import Data.Text (Text, intersperse, pack)
+import Data.Text (Text, pack)
 import qualified Data.Text as Text
 import Data.Void (Void)
-import Text.Megaparsec (Parsec, between, empty, errorBundlePretty, manyTill, notFollowedBy, optional, parse, try)
-import Text.Megaparsec.Char (alphaNumChar, char, letterChar, space1, string, symbolChar)
-import qualified Text.Megaparsec.Char.Lexer as L
 import Minilang.Lambda.Eval (Term)
 import qualified Minilang.Lambda.Eval as Eval
+import Text.Megaparsec (Parsec, between, empty, errorBundlePretty, parse, try)
+import Text.Megaparsec.Char (alphaNumChar, char, letterChar, space1, symbolChar)
+import qualified Text.Megaparsec.Char.Lexer as L
 
 type Parser = Parsec Void Text
 
@@ -24,7 +25,7 @@ data AST = Sym Text | Abs [Text] AST | App AST AST [AST]
 
 desugar :: AST -> Term
 desugar = \case
-  Sym sym -> Eval.Var sym
+  Sym symb -> Eval.Var symb
   Abs bound body ->
     foldr
       Eval.Lam
@@ -38,7 +39,7 @@ desugar = \case
 
 pretty :: AST -> Text
 pretty = \case
-  Sym sym -> sym
+  Sym symb -> symb
   Abs bound body -> "(lam (" <> Text.unwords bound <> ") " <> pretty body <> ")"
   App x y rest -> "(" <> mconcat (List.intersperse " " (pretty <$> (x : y : rest))) <> ")"
 
@@ -47,7 +48,7 @@ parse =
   first (ParseError . pack . errorBundlePretty) . Text.Megaparsec.parse ast ""
 
 ast :: Parser AST
-ast = try lambda <|> try sym <|> try app
+ast = try letExpr <|> try lambda <|> try sym <|> try app
 
 app :: Parser AST
 app =
@@ -67,6 +68,24 @@ lambda =
            )
     )
 
+letExpr :: Parser AST
+letExpr =
+  between
+    lpar
+    rpar
+    ( do
+        void $ lexeme "let"
+        (bound, binding) <-
+          between
+            lpar
+            rpar
+            ( (,) <$> identifier <*> ast
+            )
+        body <- ast
+        let fun = Abs [pack bound] body
+        return $ App fun binding []
+    )
+
 lpar :: Parser Text
 lpar = symbol "("
 
diff --git a/lambda-calcul/haskell/test/Minilang/Lambda/ParserSpec.hs b/lambda-calcul/haskell/test/Minilang/Lambda/ParserSpec.hs
index 5d3e8ab..edd39df 100644
--- a/lambda-calcul/haskell/test/Minilang/Lambda/ParserSpec.hs
+++ b/lambda-calcul/haskell/test/Minilang/Lambda/ParserSpec.hs
@@ -58,6 +58,14 @@ spec = parallel $ do
      in nestedApp term === ar
           & counterexample ("Desugared term: " <> show term)
 
+  prop "parses a let-binding as a function application" $ \(Identifier ident) ->
+    forAll (genAst 3) $ \binding ->
+      forAll (genAst 2) $ \body ->
+        let fun = Abs [ident] body
+            app = App fun binding []
+         in parse ("(let (" <> ident <> " " <> pretty binding <> ") " <> pretty body <> ")")
+              `shouldBe` Right app
+
 spine :: AST -> Int
 spine = \case
   App x y rest -> 1 + spine x + length rest