Optimize mapOptional and add more efficient findOptional (#9214)

* Optimize mapOptional and add more efficient findOptional

Given that we don’t have list fusion catOptionals . map f is clearly
less efficient than this version.

I also added findOptional which can be pretty handy in certain cases.

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* more foldr

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* fix tests

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compiler/damlc/daml-stdlib-src/DA/Optional.daml

@@ -68,10 +68,24 @@ isNone = not . isSome
 -- the result list. If it is `Some b`, then `b` is included in the
 -- result list.
 mapOptional : (a -> Optional b) -> [a] -> [b]
-mapOptional f = catOptionals . map f
+mapOptional p = foldr f []
+  where f x acc = case p x of
+          None -> acc
+          Some y -> y :: acc
 
 -- | Perform some operation on `Some`, given the field inside the
 -- `Some`.
 whenSome : Applicative m => Optional a -> (a -> m ()) -> m ()
 whenSome None _ = pure ()
 whenSome (Some v) f = f v
+
+-- | The `findOptional` returns the value of the predicate at the first
+-- element where it returns `Some`. `findOptional` is similar to `find` but it
+-- allows you to return a value from the predicate. This is useful both as a more
+-- type safe version if the predicate corresponds to a pattern match
+-- and for performance to avoid duplicating work performed in the predicate.
+findOptional : (a -> Optional b) -> [a] -> Optional b
+findOptional _ [] = None
+findOptional p (x :: xs) = case p x of
+  None -> findOptional p xs
+  Some y -> Some y

compiler/damlc/tests/daml-test-files/Optional.daml

@@ -76,3 +76,13 @@ testWhenSome = scenario do
   submit p (fetch cid)
   whenSome (Some 5) (\x -> submit p $ exercise cid $ ConsumeIfPositive x cid)
   submitMustFail p (fetch cid)
+
+testFindOptional = scenario do
+  findOptional toLeft [] === (None : Optional ())
+  findOptional toLeft [Right 1] === (None : Optional ())
+  findOptional toLeft [Right 1, Left 2] === Some 2
+  findOptional toLeft [Right 1, Left 2, Left 3] === Some 2
+
+toLeft : Either a b -> Optional a
+toLeft (Left a) = Some a
+toLeft (Right _) = None