Branch the dropLast functionality from dropAt, inserting an index gathered by subtracting 1 from the list length

compiler/builtins/docs/List.roc

@@ -34,6 +34,7 @@ interface List
             sortWith,
             drop,
             dropAt,
+            dropLast,
             swap
         ]
     imports []
@@ -439,6 +440,9 @@ drop : List elem, Nat -> List elem
 ## To replace the element at a given index, instead of dropping it, see [List.set].
 dropAt : List elem, Nat -> List elem
 
+## Drops the last element in a List.
+dropLast : List elem -> List elem
+
 ## Adds a new element to the end of the list.
 ##
 ## >>> List.append [ "a", "b" ] "c"

compiler/builtins/src/bitcode.rs

@@ -177,6 +177,7 @@ pub const LIST_APPEND: &str = "roc_builtins.list.append";
 pub const LIST_PREPEND: &str = "roc_builtins.list.prepend";
 pub const LIST_DROP: &str = "roc_builtins.list.drop";
 pub const LIST_DROP_AT: &str = "roc_builtins.list.drop_at";
+pub const LIST_DROP_LAST: &str = "roc_builtins.list.drop_last";
 pub const LIST_SWAP: &str = "roc_builtins.list.swap";
 pub const LIST_SINGLE: &str = "roc_builtins.list.single";
 pub const LIST_JOIN: &str = "roc_builtins.list.join";

compiler/can/src/builtins.rs

@@ -2004,6 +2004,45 @@ fn list_drop_at(symbol: Symbol, var_store: &mut VarStore) -> Def {
     )
 }
 
+/// List.dropLast: List elem -> List elem
+fn list_drop_last(symbol: Symbol, var_store: &mut VarStore) -> Def {
+    let list_var = var_store.fresh();
+
+    let body = RunLowLevel {
+        op: LowLevel::ListDropAt,
+        args: vec![
+            (list_var, Var(Symbol::ARG_1)),
+            (index_var,
+                // Num.sub (List.len list) 1
+                RunLowLevel {
+                    op: LowLevel::NumSubWrap,
+                    args: vec![
+                        (
+                            arg_var,
+                            // List.len list
+                            RunLowLevel {
+                                op: LowLevel::ListLen,
+                                args: vec![(list_var, Var(Symbol::ARG_1))],
+                                ret_var: len_var,
+                            },
+                        ),
+                        (arg_var, int(num_var, num_precision_var, 1)),
+                    ],
+                    ret_var: len_var,
+                },
+            ),
+        ],
+        ret_var: list_var,
+    };
+
+    defn(
+        symbol,
+        vec![(list_var, Symbol::ARG_1), (index_var, Symbol::ARG_2)],
+        var_store,
+        body,
+        list_var,
+    )
+}
 /// List.append : List elem, elem -> List elem
 fn list_append(symbol: Symbol, var_store: &mut VarStore) -> Def {
     let list_var = var_store.fresh();

compiler/module/src/low_level.rs

@@ -42,7 +42,6 @@ pub enum LowLevel {
     ListSortWith,
     ListDrop,
     ListDropAt,
-    ListDropLast,
     ListSwap,
     DictSize,
     DictEmpty,
@@ -130,7 +129,6 @@ macro_rules! first_order {
             | ListSet
             | ListDrop
             | ListDropAt
-            | ListDropLast
             | ListSingle
             | ListRepeat
             | ListReverse