From 78d2b3acf77dc0500d27836915dce99a9c368b53 Mon Sep 17 00:00:00 2001
From: Arnaud Bailly <arnaud@pankzsoft.com>
Date: Fri, 17 Oct 2025 16:36:40 +0200
Subject: feat: can parse a simple λ-expression
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---
 .../haskell/src/Minilang/Lambda/Parser.hs          | 31 +++++++++++++++++++---
 .../haskell/test/Minilang/Lambda/ParserSpec.hs     |  2 ++
 2 files changed, 29 insertions(+), 4 deletions(-)

diff --git a/lambda-calcul/haskell/src/Minilang/Lambda/Parser.hs b/lambda-calcul/haskell/src/Minilang/Lambda/Parser.hs
index da590ad..545536d 100644
--- a/lambda-calcul/haskell/src/Minilang/Lambda/Parser.hs
+++ b/lambda-calcul/haskell/src/Minilang/Lambda/Parser.hs
@@ -13,15 +13,38 @@ type Parser = Parsec Void Text
 data ParseError = ParseError Text
   deriving (Eq, Show)
 
-data AST = Sym Text
+data AST = Sym Text | Abs Text AST
   deriving (Eq, Show)
 
 parse :: Text -> Either ParseError AST
 parse =
-  first (ParseError . pack . errorBundlePretty) . Text.Megaparsec.parse symbol ""
+  first (ParseError . pack . errorBundlePretty) . Text.Megaparsec.parse ast ""
 
-symbol :: Parser AST
-symbol = Sym . pack <$> identifier
+ast :: Parser AST
+ast = try lambda <|> try sym
+
+lambda :: Parser AST
+lambda =
+  between
+    lpar
+    rpar
+    ( do
+        lexeme "lam"
+        binding <- pack <$> between lpar rpar identifier
+        Abs binding <$> ast
+    )
+
+lpar :: Parser Text
+lpar = symbol "("
+
+rpar :: Parser Text
+rpar = symbol ")"
+
+symbol :: Text -> Parser Text
+symbol = L.symbol spaceConsumer
+
+sym :: Parser AST
+sym = Sym . pack <$> identifier
 
 identifier :: Parser String
 identifier =
diff --git a/lambda-calcul/haskell/test/Minilang/Lambda/ParserSpec.hs b/lambda-calcul/haskell/test/Minilang/Lambda/ParserSpec.hs
index 4d6f6e0..c1eab27 100644
--- a/lambda-calcul/haskell/test/Minilang/Lambda/ParserSpec.hs
+++ b/lambda-calcul/haskell/test/Minilang/Lambda/ParserSpec.hs
@@ -10,6 +10,8 @@ spec :: Spec
 spec = parallel $ do
   prop "parses an identifier as a variable" $ \(Identifier ident) ->
     parse ident `shouldBe` Right (Sym ident)
+  prop "parses a lambda-expression as an abstraction" $ \(Identifier ident) (Identifier body) ->
+    parse ("(lam (" <> ident <> ") " <> body <> ")") `shouldBe` Right (Abs ident (Sym body))
 
 newtype Identifier = Identifier Text
   deriving (Eq, Show)
-- 
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