From a40b7da1ab4028bba53f9768b7a58a7c619a052d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Roman=20Smr=C5=BE?= <roman.smrz@seznam.cz>
Date: Mon, 18 May 2026 20:34:43 +0200
Subject: Concatenation operator for lists

Changelog: Added `++` operator to concatenate lists
---
 README.md                 |  6 +++++-
 src/Parser/Expr.hs        | 24 ++++++++++++++++++++++++
 test/asset/list/concat.et |  8 ++++++++
 test/script/list.et       |  8 ++++++++
 4 files changed, 45 insertions(+), 1 deletion(-)

diff --git a/README.md b/README.md
index 8d40a6f..62ae1a0 100644
--- a/README.md
+++ b/README.md
@@ -236,7 +236,11 @@ let numbers = [1, 2, 4]
 List elements can be of any type, but all elements of a particular list must have the same type.
 They can be concatenated using the `concat` function, which takes a list of lists as argument:
 ```
-let list = concat [[1], [2, 3], [4]] # = [1, 2, 3, 4,]
+let list = concat [[1], [2, 3], [4]]  # = [1, 2, 3, 4]
+```
+Or with the `++` operator:
+```
+let list = [1] ++ [2, 3] ++ [4]  # = [1, 2, 3, 4]
 ```
 
 ### Built-in commands
diff --git a/src/Parser/Expr.hs b/src/Parser/Expr.hs
index 7d57791..a0ae70d 100644
--- a/src/Parser/Expr.hs
+++ b/src/Parser/Expr.hs
@@ -302,6 +302,19 @@ someExpr complexity = label "expression" $ do
                              , SomeBinOp ((-) @Scientific)
                              ]
               ]
+            , [ let tvar = TypeVar "a"
+                    targs = FunctionArguments $ M.fromList
+                        [ ( Just "$l", ( VarName "$l", SomeArgumentType RequiredArgument $ ExprTypeApp (ExprTypeConstr1 (Proxy @[])) [ ExprTypeVar tvar ]) )
+                        , ( Just "$r", ( VarName "$r", SomeArgumentType RequiredArgument $ ExprTypeApp (ExprTypeConstr1 (Proxy @[])) [ ExprTypeVar tvar ]) )
+                        ]
+                 in infixrExpr "++" $ SomeExpr $ TypeLambda tvar (ExprTypeFunction (ExprTypeArguments $ fmap snd targs) (ExprTypeApp (ExprTypeConstr1 (Proxy @[])) [ ExprTypeVar tvar ])) $ \case
+                        ExprTypePrim (Proxy :: Proxy a) ->
+                            HideFunType (fmap snd targs) $ ArgsReq targs $
+                                FunctionAbstraction $ ((++) @a)
+                                    <$> (Variable SourceLineBuiltin $ LocalVarName $ VarName "$l")
+                                    <*> (Variable SourceLineBuiltin $ LocalVarName $ VarName "$r")
+                        t -> Undefined ("ambiguous type ‘" <> T.unpack (textSomeExprType t) <> "’ for operator ‘++’") :: Expr DynamicType
+              ]
             , [ binary' "==" (\op xs ys -> length xs == length ys && and (zipWith op xs ys)) $
                               [ SomeBinOp ((==) @Integer)
                               , SomeBinOp ((==) @Scientific)
@@ -345,6 +358,17 @@ someExpr complexity = label "expression" $ do
                 choice $ map (\(SomeUnOp op) -> SomeExpr <$> applyUnOp off op e) ops
 
 
+    infixrExpr :: String -> SomeExpr -> Operator TestParser (TestParser SomeExpr)
+    infixrExpr name fun = InfixR $ do
+        void $ osymbol name
+        return $ \p q -> do
+            loff <- stateOffset <$> getParserState
+            l <- p
+            roff <- stateOffset <$> getParserState
+            r <- q
+            applyFunctionArguments (FunctionArguments $ M.fromList [ ( Just "$l", ( loff, l ) ), ( Just "$r", ( roff, r ) ) ]) fun
+
+
     binary :: String -> [SomeBinOp] -> Operator TestParser (TestParser SomeExpr)
     binary name = binary' name (undefined :: forall a b. (a -> b -> Void) -> [a] -> [b] -> Integer)
       -- use 'Void' that can never match actually used type to disable recursion
diff --git a/test/asset/list/concat.et b/test/asset/list/concat.et
index 613df57..c669a78 100644
--- a/test/asset/list/concat.et
+++ b/test/asset/list/concat.et
@@ -28,3 +28,11 @@ test Test:
     local:
         shell on n:
             echo "c3-end"
+
+    let c4 = list1 ++ list2 ++ [ 6, 5 ] ++ list2
+    for i in c4:
+        shell on n:
+            echo "c4 $i"
+    local:
+        shell on n:
+            echo "c4-end"
diff --git a/test/script/list.et b/test/script/list.et
index a654111..cfd4803 100644
--- a/test/script/list.et
+++ b/test/script/list.et
@@ -28,6 +28,14 @@ test ListConcat:
             "c3 2"
             "c3 3"
             "c3-end"
+            "c4 1"
+            "c4 2"
+            "c4 3"
+            "c4 6"
+            "c4 5"
+            "c4 2"
+            "c4 3"
+            "c4-end"
         local:
             expect /(run-.*)/ capture done
             guard (done == "run-done")
-- 
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