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filterA

Implemented a filterA function using the approach borrowed from mapA in the standard library (and the filterM in Haskell).

Given the "classic" Iou example if one has a collection of ContractId Ious and is interested in only the "large ones" he could use this new filter like this:

filter_ious: Scenario ()
filter_ious = scenario do
  issuer <- getParty "John Doe"

  let
    usd = Iou issuer issuer "USD" 1.23 []
    gbp = Iou issuer issuer "GBP" 2.34 []
    eur = Iou issuer issuer "EUR" 3.45 []
    ious = [usd, gbp, eur]

    largeIou iouId = do
      iou <- fetch iouId
      pure $ iou.amount > (2.0 : Decimal)

  iouIds <- issuer `submit` forA ious create

  largeIds <- issuer `submit` filterA largeIou iouIds

  largeIous <- issuer `submit` forA largeIds fetch
  [gbp, eur] === largeIous