List.join builtin and unification

compiler/builtins/src/std.rs

@@ -528,6 +528,15 @@ pub fn types() -> MutMap<Symbol, (SolvedType, Region)> {
         ),
     );
 
+    // join : List (List elem) -> List elem
+    add_type(
+        Symbol::LIST_JOIN,
+        SolvedType::Func(
+            vec![list_type(list_type(flex(TVAR1)))],
+            Box::new(list_type(flex(TVAR1))),
+        ),
+    );
+
     // single : a -> List a
     add_type(
         Symbol::LIST_SINGLE,

compiler/can/src/builtins.rs

@@ -61,6 +61,7 @@ pub fn builtin_defs(var_store: &mut VarStore) -> MutMap<Symbol, Def> {
         Symbol::LIST_REVERSE => list_reverse,
         Symbol::LIST_CONCAT => list_concat,
         Symbol::LIST_PREPEND => list_prepend,
+        Symbol::LIST_JOIN => list_join,
         Symbol::NUM_ADD => num_add,
         Symbol::NUM_SUB => num_sub,
         Symbol::NUM_MUL => num_mul,
@@ -903,6 +904,26 @@ fn list_prepend(symbol: Symbol, var_store: &mut VarStore) -> Def {
     )
 }
 
+/// List.join : List (List elem) -> List elem
+fn list_join(symbol: Symbol, var_store: &mut VarStore) -> Def {
+    let list_var = var_store.fresh();
+    let list_of_list_var = var_store.fresh();
+
+    let body = RunLowLevel {
+        op: LowLevel::ListJoin,
+        args: vec![(list_of_list_var, Var(Symbol::ARG_1))],
+        ret_var: list_var,
+    };
+
+    defn(
+        symbol,
+        vec![(list_of_list_var, Symbol::ARG_1)],
+        var_store,
+        body,
+        list_var,
+    )
+}
+
 /// Num.rem : Int, Int -> Result Int [ DivByZero ]*
 fn num_rem(symbol: Symbol, var_store: &mut VarStore) -> Def {
     let num_var = var_store.fresh();

compiler/gen/src/llvm/build.rs

@@ -1533,6 +1533,11 @@ fn list_push<'a, 'ctx, 'env>(
     )
 }
 
+/// List.join : List (List elem) -> List elem
+fn list_join<'a, 'ctx, 'env>(env: &Env<'a, 'ctx, 'env>) -> BasicValueEnum<'ctx> {
+    empty_list(env)
+}
+
 /// List.prepend List elem, elem -> List elem
 fn list_prepend<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
@@ -1928,6 +1933,12 @@ fn run_low_level<'a, 'ctx, 'env>(
 
             list_prepend(env, original_wrapper, elem, elem_layout)
         }
+        ListJoin => {
+            // List.join : List (List elem) -> List elem
+            debug_assert_eq!(args.len(), 1);
+
+            list_join(env)
+        }
         NumAbs | NumNeg | NumRound | NumSqrtUnchecked | NumSin | NumCos | NumToFloat => {
             debug_assert_eq!(args.len(), 1);
 

compiler/gen/tests/gen_list.rs

@@ -98,6 +98,12 @@ mod gen_list {
         );
     }
 
+    #[test]
+    fn list_join() {
+        assert_evals_to!("List.join []", &[], &'static [i64]);
+        assert_evals_to!("List.join [ [ 1 ] ]", &[1], &'static [i64]);
+    }
+
     #[test]
     fn list_single() {
         assert_evals_to!("List.single 1", &[1], &'static [i64]);

compiler/module/src/low_level.rs

@@ -13,6 +13,7 @@ pub enum LowLevel {
     ListConcat,
     ListAppend,
     ListPrepend,
+    ListJoin,
     NumAdd,
     NumSub,
     NumMul,

compiler/module/src/symbol.rs

@@ -659,6 +659,7 @@ define_builtins! {
         13 LIST_REPEAT: "repeat"
         14 LIST_REVERSE: "reverse"
         15 LIST_PREPEND: "prepend"
+        16 LIST_JOIN: "join"
     }
     5 RESULT: "Result" => {
         0 RESULT_RESULT: "Result" imported // the Result.Result type alias