Add builtin List.split

compiler/builtins/bitcode/src/list.zig

@@ -870,6 +870,9 @@ pub fn listSublist(
     len: usize,
     dec: Dec,
 ) callconv(.C) RocList {
+    if (len == 0) {
+        return RocList.empty();
+    }
     if (list.bytes) |source_ptr| {
         const size = list.len();
 

compiler/builtins/src/std.rs

@@ -1015,6 +1015,25 @@ pub fn types() -> MutMap<Symbol, (SolvedType, Region)> {
         Box::new(list_type(flex(TVAR1))),
     );
 
+    // split : List elem, Nat -> { before: List elem, others: List elem }
+    add_top_level_function_type!(
+        Symbol::LIST_SPLIT,
+        vec![list_type(flex(TVAR1)), nat_type(),],
+        Box::new(SolvedType::Record {
+            fields: vec![
+                (
+                    "before".into(),
+                    RecordField::Required(list_type(flex(TVAR1)))
+                ),
+                (
+                    "others".into(),
+                    RecordField::Required(list_type(flex(TVAR1)))
+                ),
+            ],
+            ext: Box::new(SolvedType::EmptyRecord),
+        },),
+    );
+
     // drop : List elem, Nat -> List elem
     add_top_level_function_type!(
         Symbol::LIST_DROP,

compiler/can/src/builtins.rs

@@ -1,9 +1,9 @@
 use crate::def::Def;
 use crate::expr::{ClosureData, Expr::*};
-use crate::expr::{Expr, Recursive, WhenBranch};
+use crate::expr::{Expr, Field, Recursive, WhenBranch};
 use crate::pattern::Pattern;
 use roc_collections::all::SendMap;
-use roc_module::ident::TagName;
+use roc_module::ident::{Lowercase, TagName};
 use roc_module::low_level::LowLevel;
 use roc_module::operator::CalledVia;
 use roc_module::symbol::Symbol;
@@ -96,6 +96,7 @@ pub fn builtin_defs_map(symbol: Symbol, var_store: &mut VarStore) -> Option<Def>
         LIST_TAKE_FIRST => list_take_first,
         LIST_TAKE_LAST => list_take_last,
         LIST_SUBLIST => list_sublist,
+        LIST_SPLIT => list_split,
         LIST_DROP => list_drop,
         LIST_DROP_AT => list_drop_at,
         LIST_DROP_FIRST => list_drop_first,
@@ -2147,6 +2148,74 @@ fn list_sublist(symbol: Symbol, var_store: &mut VarStore) -> Def {
     )
 }
 
+/// List.split : List elem, Nat -> { before: List elem, others: List elem }
+fn list_split(symbol: Symbol, var_store: &mut VarStore) -> Def {
+    let list_var = var_store.fresh();
+    let index_var = var_store.fresh();
+
+    let sym_list = Symbol::ARG_1;
+    let sym_index = Symbol::ARG_2;
+
+    let ret_var = var_store.fresh();
+    let zero = int(index_var, Variable::NATURAL, 0);
+
+    let get_before = RunLowLevel {
+        op: LowLevel::ListSublist,
+        args: vec![
+            (list_var, Var(sym_list)),
+            (index_var, zero),
+            (index_var, Var(sym_index)),
+        ],
+        ret_var: list_var,
+    };
+
+    let get_list_len = RunLowLevel {
+        op: LowLevel::ListLen,
+        args: vec![(list_var, Var(sym_list))],
+        ret_var: index_var,
+    };
+
+    let get_others_len = RunLowLevel {
+        op: LowLevel::NumSubWrap,
+        args: vec![(index_var, get_list_len), (index_var, Var(sym_index))],
+        ret_var: index_var,
+    };
+
+    let get_others = RunLowLevel {
+        op: LowLevel::ListSublist,
+        args: vec![
+            (list_var, Var(sym_list)),
+            (index_var, Var(sym_index)),
+            (index_var, get_others_len),
+        ],
+        ret_var: list_var,
+    };
+
+    let before = Field {
+        var: list_var,
+        region: Region::zero(),
+        loc_expr: Box::new(no_region(get_before)),
+    };
+    let others = Field {
+        var: list_var,
+        region: Region::zero(),
+        loc_expr: Box::new(no_region(get_others)),
+    };
+
+    let body = record(
+        vec![("before".into(), before), ("others".into(), others)],
+        var_store,
+    );
+
+    defn(
+        symbol,
+        vec![(list_var, sym_list), (index_var, sym_index)],
+        var_store,
+        body,
+        ret_var,
+    )
+}
+
 /// List.drop : List elem, Nat -> List elem
 fn list_drop(symbol: Symbol, var_store: &mut VarStore) -> Def {
     let list_var = var_store.fresh();
@@ -4296,17 +4365,17 @@ fn tag(name: &'static str, args: Vec<Expr>, var_store: &mut VarStore) -> Expr {
     }
 }
 
-// #[inline(always)]
-// fn record(fields: Vec<(Lowercase, Field)>, var_store: &mut VarStore) -> Expr {
-// let mut send_map = SendMap::default();
-// for (k, v) in fields {
-// send_map.insert(k, v);
-// }
-// Expr::Record {
-// record_var: var_store.fresh(),
-// fields: send_map,
-// }
-// }
+#[inline(always)]
+fn record(fields: Vec<(Lowercase, Field)>, var_store: &mut VarStore) -> Expr {
+    let mut send_map = SendMap::default();
+    for (k, v) in fields {
+        send_map.insert(k, v);
+    }
+    Expr::Record {
+        record_var: var_store.fresh(),
+        fields: send_map,
+    }
+}
 
 #[inline(always)]
 fn defn(

compiler/module/src/symbol.rs

@@ -1072,6 +1072,7 @@ define_builtins! {
         47 LIST_FIND: "find"
         48 LIST_FIND_RESULT: "#find_result" // symbol used in the definition of List.find
         49 LIST_SUBLIST: "sublist"
+        50 LIST_SPLIT: "split"
     }
     5 RESULT: "Result" => {
         0 RESULT_RESULT: "Result" imported // the Result.Result type alias

compiler/solve/tests/solve_expr.rs

@@ -3793,6 +3793,14 @@ mod solve_expr {
         );
     }
 
+    #[test]
+    fn list_split() {
+        infer_eq_without_problem(
+            indoc!("List.split"),
+            "List a, Nat -> { before : List a, others : List a }",
+        );
+    }
+
     #[test]
     fn list_drop_last() {
         infer_eq_without_problem(

compiler/test_gen/src/gen_list.rs

@@ -248,6 +248,47 @@ fn list_sublist() {
     );
 }
 
+#[test]
+#[cfg(any(feature = "gen-llvm"))]
+fn list_split() {
+    assert_evals_to!(
+        r#"
+               list = List.split [1, 2, 3] 0
+               list.before
+            "#,
+        RocList::from_slice(&[]),
+        RocList<i64>
+    );
+    assert_evals_to!(
+        r#"
+               list = List.split [1, 2, 3] 0
+               list.others
+            "#,
+        RocList::from_slice(&[1, 2, 3]),
+        RocList<i64>
+    );
+
+    assert_evals_to!(
+        "List.split [1, 2, 3] 1",
+        (RocList::from_slice(&[1]), RocList::from_slice(&[2, 3]),),
+        (RocList<i64>, RocList<i64>,)
+    );
+    assert_evals_to!(
+        "List.split [1, 2, 3] 3",
+        (RocList::from_slice(&[1, 2, 3]), RocList::from_slice(&[]),),
+        (RocList<i64>, RocList<i64>,)
+    );
+    assert_evals_to!(
+        "List.split [1, 2, 3] 4",
+        (RocList::from_slice(&[1, 2, 3]), RocList::from_slice(&[]),),
+        (RocList<i64>, RocList<i64>,)
+    );
+    assert_evals_to!(
+        "List.split [] 1",
+        (RocList::from_slice(&[]), RocList::from_slice(&[]),),
+        (RocList<i64>, RocList<i64>,)
+    );
+}
 #[test]
 #[cfg(any(feature = "gen-llvm"))]
 fn list_drop() {