From 3b20f8a9e85696b86f698e1bdbdbd31b786b77c2 Mon Sep 17 00:00:00 2001
From: Arnaud Bailly <arnaud@pankzsoft.com>
Date: Fri, 17 Oct 2025 16:49:32 +0200
Subject: feat: parses single application

---
 lambda-calcul/haskell/src/Minilang/Lambda/Parser.hs      | 11 +++++++++--
 lambda-calcul/haskell/test/Minilang/Lambda/ParserSpec.hs |  3 +++
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/lambda-calcul/haskell/src/Minilang/Lambda/Parser.hs b/lambda-calcul/haskell/src/Minilang/Lambda/Parser.hs
index 8e97d46..e60d761 100644
--- a/lambda-calcul/haskell/src/Minilang/Lambda/Parser.hs
+++ b/lambda-calcul/haskell/src/Minilang/Lambda/Parser.hs
@@ -13,7 +13,7 @@ type Parser = Parsec Void Text
 data ParseError = ParseError Text
   deriving (Eq, Show)
 
-data AST = Sym Text | Abs [Text] AST
+data AST = Sym Text | Abs [Text] AST | App AST AST
   deriving (Eq, Show)
 
 parse :: Text -> Either ParseError AST
@@ -21,7 +21,14 @@ parse =
   first (ParseError . pack . errorBundlePretty) . Text.Megaparsec.parse ast ""
 
 ast :: Parser AST
-ast = try lambda <|> try sym
+ast = try lambda <|> try sym <|> try app
+
+app :: Parser AST
+app =
+  between
+    lpar
+    rpar
+    (App <$> ast <*> ast)
 
 lambda :: Parser AST
 lambda =
diff --git a/lambda-calcul/haskell/test/Minilang/Lambda/ParserSpec.hs b/lambda-calcul/haskell/test/Minilang/Lambda/ParserSpec.hs
index ea7cd89..b52017a 100644
--- a/lambda-calcul/haskell/test/Minilang/Lambda/ParserSpec.hs
+++ b/lambda-calcul/haskell/test/Minilang/Lambda/ParserSpec.hs
@@ -20,6 +20,9 @@ spec = parallel $ do
         nestedAbs = Abs vars (Sym body)
      in parse ("(lam (" <> Text.unwords vars <> ") " <> body <> ")") `shouldBe` Right nestedAbs
 
+  prop "parses an application" $ \(Identifier ident1) (Identifier ident2) ->
+    parse ("(" <> ident1 <> " " <> ident2 <> ")") `shouldBe` Right (App (Sym ident1) (Sym ident2))
+
 newtype Identifier = Identifier {unIdent :: Text}
   deriving (Eq, Show)
 
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