Add filterA to Prelude (#5809)

* CHANGELOG_BEGIN

Added filterA to Prelude.

* Update compiler/damlc/daml-stdlib-src/DA/Internal/Prelude.daml

Co-authored-by: Shayne Fletcher <shayne@shaynefletcher.org>

* Removed not so useful comment.

* Moved filterA from Prelude to Action.

* filterA is a one-liner now.

* Provided more meaningful example to filterA.

* Added test for filterA.

* Removed failing doctest.
CHANGELOG_END

Co-authored-by: Shayne Fletcher <shayne@shaynefletcher.org>

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

@@ -59,6 +59,15 @@ foldl1A : Action m => (a -> a -> m a) -> [a] -> m a
 foldl1A f (x :: xs) = foldlA f x xs
 foldl1A _ [] = error "foldl1M: empty list"
 
+-- | Filters the list using the applicative function: keeps only the elements where the predicate holds.
+-- Example: given a collection of Iou contract IDs one can find only the GBPs.
+--
+-- ```
+-- filterA (fmap (\iou -> iou.currency == "GBP") . fetch) iouCids
+-- ```
+filterA : Applicative m => (a -> m Bool) -> [a] -> m [a]
+filterA p = foldr (\x -> liftA2 (\pHolds -> if pHolds then (x ::) else identity) (p x)) (pure [])
+
 -- | `replicateA n act` performs the action `n` times, gathering the
 --  results.
 replicateA : (Applicative m) => Int -> m a -> m [a]

compiler/damlc/daml-stdlib-src/DA/Internal/Prelude.daml

@@ -624,7 +624,7 @@ null : [a] -> Bool
 null [] = True
 null _ = False
 
--- | Filter the list using the function: keep only the elements where the predicate holds.
+-- | Filters the list using the function: keep only the elements where the predicate holds.
 filter : (a -> Bool) -> [a] -> [a]
 filter p = foldr (\x xs -> if p x then x :: xs else xs) []
 

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

@@ -0,0 +1,31 @@
+-- Copyright (c) 2020, Digital Asset (Switzerland) GmbH and/or its affiliates.
+-- All rights reserved.
+
+module ActionTest where
+
+import DA.Assert
+import DA.Action
+
+import Iou12
+
+testFilterA: Scenario ()
+testFilterA = scenario do
+  bank <- getParty "Acme Bank"
+  alice <- getParty "Alice"
+  bob <- getParty "Bob"
+  charlie <- getParty "Charlie"
+  let
+    aliceIou = Iou bank alice "USD" 1.23 []
+    bobIou = Iou bank bob "GBP" 2.34 []
+    charlieIou = Iou bank charlie "EUR" 3.45 []
+    ious = [aliceIou, bobIou, charlieIou]
+
+  iouIds <- bank `submit` forA ious create
+
+  aliceIouIds <- bank `submit` filterA (fmap (\iou -> iou.owner == alice) . fetch) iouIds
+  aliceIous <- bank `submit` forA aliceIouIds fetch
+  [aliceIou] === aliceIous
+
+  largeIouIds <- bank `submit` filterA (fmap (\iou -> iou.amount > (2.0 : Decimal)) . fetch) iouIds
+  largeIous <- bank `submit` forA largeIouIds fetch
+  [bobIou, charlieIou] === largeIous