Add List.walk_try!

crates/compiler/builtins/roc/List.roc

@@ -75,6 +75,7 @@ module [
     for_each!,
     for_each_try!,
     walk!,
+    walk_try!,
 ]
 
 import Bool exposing [Bool, Eq]
@@ -1514,7 +1515,39 @@ for_each_try! = \list, func! ->
 walk! : List elem, state, (state, elem => state) => state
 walk! = \list, state, func! ->
     when list is
-        [] -> state
+        [] ->
+            state
+
         [elem, .. as rest] ->
             next_state = func!(state, elem)
             walk!(rest, next_state, func!)
+
+## Build a value from the contents of a list, using an effectful function that might fail.
+##
+## If the function returns `Err`, the iteration stops and the error is returned.
+##
+## ```
+## names = try List.walk_try!(
+##         ["First", "Middle", "Last"],
+##         [],
+##         \accumulator, which ->
+##             try Stdout.write! ("$(which) name: ")
+##             name = try Stdin.line! ({})
+##             Ok (List.append accumulator name),
+##     )
+## ```
+##
+## This is the same as [walk_try], except that the step function can have effects.
+walk_try! : List elem, state, (state, elem => Result state err) => Result state err
+walk_try! = \list, state, func! ->
+    when list is
+        [] ->
+            Ok(state)
+
+        [elem, .. as rest] ->
+            when func!(state, elem) is
+                Ok(next_state) ->
+                    walk_try!(rest, next_state, func!)
+
+                Err(err) ->
+                    Err(err)

crates/compiler/module/src/symbol.rs

@@ -1515,6 +1515,7 @@ define_builtins! {
         92 LIST_WALK_FX: "walk!"
         93 LIST_SPLIT_ON: "split_on"
         94 LIST_SPLIT_ON_LIST: "split_on_list"
+        95 LIST_WALK_TRY_FX: "walk_try!"
     }
     7 RESULT: "Result" => {
         0 RESULT_RESULT: "Result" exposed_type=true // the Result.Result type alias