Add List.mapTry

crates/compiler/builtins/roc/List.roc

@@ -38,6 +38,7 @@ interface List
         min,
         max,
         map4,
+        mapTry,
         dropFirst,
         joinMap,
         any,
@@ -858,6 +859,32 @@ split = \elements, userSplitIndex ->
 
     { before, others }
 
+## Like [List.map], except the transformation function returns a [Result].
+## If that function ever returns `Err`, [mapTry] immediately returns `Err`.
+## If it returns `Ok` for every element, [mapTry] returns `Ok` with the transformed list.
+mapTry : List elem, (elem -> Result ok err) -> Result (List ok) err
+mapTry = \list, toResult ->
+    walkTry list [] \state, elem ->
+        Result.map (toResult elem) \ok ->
+            List.append state ok
+
+## This is the same as `iterate` but with Result instead of [Continue, Break].
+## Using `Result` saves a conditional in `mapTry`.
+## It might be useful to expose this in userspace?
+walkTry : List elem, state, (state, elem -> Result state err) -> Result state err
+walkTry = \list, init, func ->
+    walkTryHelp list init func 0 (List.len list)
+
+## internal helper
+walkTryHelp : List elem, state, (state, elem -> Result state err), Nat, Nat -> Result state err
+walkTryHelp = \list, state, f, index, length ->
+    if index < length then
+        when f state (List.getUnsafe list index) is
+            Ok nextState -> walkTryHelp list nextState f (index + 1) length
+            Err b -> Err b
+    else
+        Ok state
+
 ## Primitive for iterating over a List, being able to decide at every element whether to continue
 iterate : List elem, s, (s, elem -> [Continue s, Break b]) -> [Continue s, Break b]
 iterate = \list, init, func ->

crates/compiler/module/src/symbol.rs

@@ -1290,6 +1290,7 @@ define_builtins! {
         66 LIST_APPEND_UNSAFE: "appendUnsafe"
         67 LIST_SUBLIST_LOWLEVEL: "sublistLowlevel"
         68 LIST_CAPACITY: "capacity"
+        69 LIST_MAP_TRY: "mapTry"
     }
     7 RESULT: "Result" => {
         0 RESULT_RESULT: "Result" // the Result.Result type alias