From d41cc2a1cdf9fc7a9d5781779363b9bb3eb82c00 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Roman=20Smr=C5=BE?= <roman.smrz@seznam.cz>
Date: Sat, 25 Apr 2026 13:46:40 +0200
Subject: Concat builtin function

Changelog: Added `concat` function for lists
---
 README.md                 |  6 ++++--
 src/Test/Builtins.hs      | 13 +++++++++++++
 test/asset/list/concat.et |  8 ++++++++
 test/script/list.et       | 16 ++++++++++++++++
 4 files changed, 41 insertions(+), 2 deletions(-)
 create mode 100644 test/asset/list/concat.et
 create mode 100644 test/script/list.et

diff --git a/README.md b/README.md
index a7414b3..8d40a6f 100644
--- a/README.md
+++ b/README.md
@@ -234,8 +234,10 @@ let numbers = [1, 2, 4]
 ```
 
 List elements can be of any type, but all elements of a particular list must have the same type.
-
-Used in the `for` command.
+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,]
+```
 
 ### Built-in commands
 
diff --git a/src/Test/Builtins.hs b/src/Test/Builtins.hs
index 32483d1..85f7b86 100644
--- a/src/Test/Builtins.hs
+++ b/src/Test/Builtins.hs
@@ -6,6 +6,7 @@ import Data.Map qualified as M
 import Data.Proxy
 import Data.Scientific
 import Data.Text (Text)
+import Data.Text qualified as T
 
 import Process
 import Process.Signal
@@ -20,6 +21,7 @@ builtins = M.fromList $ concat
       , fq "guard" builtinGuard
       , fq "multiply_timeout" builtinMultiplyTimeout
       , fq "wait" builtinWait
+      , fq "concat" builtinConcat
       ]
     , map (uncurry fq) signalBuiltins
     ]
@@ -73,3 +75,14 @@ builtinMultiplyTimeout = SomeExpr $ ArgsReq (biArgs $ [ ( Just "by", SomeArgumen
 
 builtinWait :: SomeExpr
 builtinWait = SomeExpr $ Pure $ TestBlockStep EmptyTestBlock Wait
+
+builtinConcat :: SomeExpr
+builtinConcat = SomeExpr $ TypeLambda (TypeVar "a")
+    (ExprTypeFunction
+        (ExprTypeArguments $ FunctionArguments $ M.singleton Nothing $ SomeArgumentType RequiredArgument
+            (ExprTypeApp (ExprTypeConstr1 (Proxy @[])) [ ExprTypeApp (ExprTypeConstr1 (Proxy @[])) [ ExprTypeVar (TypeVar "a") ] ] ))
+        (ExprTypeApp (ExprTypeConstr1 (Proxy @[])) [ ExprTypeVar (TypeVar "a") ])
+    ) $ \case
+        ExprTypePrim (pa :: Proxy a) -> HideFunType (FunctionArguments $ M.singleton Nothing $ SomeArgumentType RequiredArgument (ExprTypePrim (Proxy :: Proxy [[ a ]]))) $
+            ArgsReq (biArgs [ ( Nothing, SomeArgumentType RequiredArgument (ExprTypePrim pa) ) ]) $ FunctionAbstraction $ (concat :: [[ a ]] -> [ a ]) <$> biVar "$0"
+        t -> Undefined ("ambiguous type ‘" <> T.unpack (textSomeExprType t) <> "’ for concat") :: Expr DynamicType
diff --git a/test/asset/list/concat.et b/test/asset/list/concat.et
new file mode 100644
index 0000000..8248cdc
--- /dev/null
+++ b/test/asset/list/concat.et
@@ -0,0 +1,8 @@
+test Test:
+    let list1 = [ 1 ]
+    let list2 = [ 2, 3 ]
+    let empty = [  ]
+
+    let c1 = concat [[ 1 ]]
+    let c2 = concat [[ 1 ], [], empty]
+    let c3 = concat [ list1, list2, [ 6, 5 ], list2 ]
diff --git a/test/script/list.et b/test/script/list.et
new file mode 100644
index 0000000..8dcb4a8
--- /dev/null
+++ b/test/script/list.et
@@ -0,0 +1,16 @@
+module list
+
+asset scripts:
+    path: ../asset/list
+
+
+test ListConcat:
+    spawn as p 
+    with p:
+        send "load ${scripts.path}/concat.et"
+        expect /load-done/
+
+        send "run Test"
+        local:
+            expect /(run-.*)/ capture done
+            guard (done == "run-done")
-- 
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