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Add Map.applyWithDefaults to satisfy Applicative laws #8

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Add Map.applyWithDefaults to satisfy Applicative laws #8

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5e4e635
Add Map.applyWithDefaults to satisfy Applicative laws
LiamGoodacre Aug 26, 2018
6d504a4
Fix Composition law of applyWithDefaults and update Identity law
LiamGoodacre Sep 2, 2018
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2 changes: 1 addition & 1 deletion src/Data/Map.purs
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Expand Up @@ -5,7 +5,7 @@ module Data.Map

import Prelude

import Data.Map.Internal (Map, alter, checkValid, delete, empty, filter, filterKeys, filterWithKey, findMax, findMin, foldSubmap, fromFoldable, fromFoldableWith, insert, isEmpty, isSubmap, lookup, lookupGE, lookupGT, lookupLE, lookupLT, member, pop, showTree, singleton, size, submap, toUnfoldable, toUnfoldableUnordered, union, unionWith, unions, update, values)
import Data.Map.Internal (Map, alter, applyWithDefault, checkValid, delete, empty, filter, filterKeys, filterWithKey, findMax, findMin, foldSubmap, fromFoldable, fromFoldableWith, insert, isEmpty, isSubmap, lookup, lookupGE, lookupGT, lookupLE, lookupLT, member, pop, showTree, singleton, size, submap, toUnfoldable, toUnfoldableUnordered, union, unionWith, unions, update, values)
import Data.Set (Set)
import Unsafe.Coerce (unsafeCoerce)

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18 changes: 17 additions & 1 deletion src/Data/Map/Internal.purs
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Expand Up @@ -37,15 +37,17 @@ module Data.Map.Internal
, filterWithKey
, filterKeys
, filter
, applyWithDefault
) where

import Prelude

import Control.Alt ((<|>))
import Data.Eq (class Eq1)
import Data.Foldable (foldl, foldMap, foldr, class Foldable)
import Data.FoldableWithIndex (class FoldableWithIndex)
import Data.FunctorWithIndex (class FunctorWithIndex, mapWithIndex)
import Data.List (List(..), (:), length, nub)
import Data.List (List(..), (:), length, nub, mapMaybe)
import Data.List.Lazy as LL
import Data.Maybe (Maybe(..), maybe, isJust, fromMaybe)
import Data.Ord (class Ord1)
Expand Down Expand Up @@ -638,3 +640,17 @@ filterKeys predicate = filterWithKey $ const <<< predicate
-- | on the value fails to hold.
filter :: forall k v. Ord k => (v -> Boolean) -> Map k v -> Map k v
filter predicate = filterWithKey $ const predicate

-- | Apply a map of functions to a map of inputs.
-- | If a key only exists on one side, then a default is used.
applyWithDefault ::
forall k i o . Ord k =>
Map k (i -> o) -> Maybe (i -> o) ->
Map k i -> Maybe i ->
Map k o
applyWithDefault fns defFn vals defVal =
let build k = Tuple k <$> ((lookup k fns <|> defFn) <*>
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Wouldn't it be possible to use union here, like the implementation in Data.Align? That way if we get a faster hedge union, this will improve for free too.

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Another thought: why not add this in the form of newtype MapWithDefault k a = MapWithDefault a (Map k a) with an Applicative instance? I think Conal has a Haskell library which does that.

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I'll have a think about the union, thanks.

Found the library you mentioned: https://hackage.haskell.org/package/total-map-0.0.6/docs/Data-TotalMap.html
I think both the type you propose and mine are useful for difference purposes.
With the one I proposed you get to model merges where one/both/neither have a default.

(lookup k vals <|> defVal)) in
let allKeys = keys (void fns <> void vals) in
fromFoldable (mapMaybe build allKeys)

38 changes: 38 additions & 0 deletions test/Test/Data/Map.purs
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Expand Up @@ -339,3 +339,41 @@ mapTests = do
quickCheck \(TestMap m :: TestMap Int Int) ->
let outList = foldrWithIndex (\i a b -> (Tuple i a) : b) Nil m
in outList == sort outList

-- Identity: (pure identity) <*> v = v
log "applyWithDefault abides applicative laws: Identity"
quickCheck \(TestMap x :: TestMap Int Int) (d :: Maybe Int) ->
let out = M.applyWithDefault M.empty (Just identity) x d
in out == x

-- Composition: pure (<<<) <*> f <*> g <*> h = f <*> (g <*> h)
log "applyWithDefault abides applicative laws: Composition"
quickCheck \(TestMap f :: TestMap Boolean (Int -> String))
fd
(TestMap g :: TestMap Boolean (Boolean -> Int))
gd
(TestMap h :: TestMap Boolean Boolean)
hd ->
let left0 = M.applyWithDefault M.empty (Just (<<<)) f fd
left1 = M.applyWithDefault left0 (Just (<<<) <*> fd) g gd
left2 = M.applyWithDefault left1 (Just (<<<) <*> fd <*> gd) h hd
right0 = M.applyWithDefault g gd h hd
right1 = M.applyWithDefault f fd right0 (gd <*> hd)
in left2 == right1

--Homomorphism: (pure f) <*> (pure x) = pure (f x)
log "applyWithDefault abides applicative laws: Homomorphism"
quickCheck \(f :: Boolean -> Int) x ->
-- we already know `Just f <*> Just x = Just (f x)`
let out = M.applyWithDefault M.empty (Just f) M.empty (Just x)
in out == (M.empty :: M.Map Int Int)

--Interchange: u <*> (pure y) = (pure (_ $ y)) <*> u
log "applyWithDefault abides applicative laws: Interchange"
quickCheck \(TestMap u :: TestMap Int (Boolean -> Int))
ud
y ->
let lhs = M.applyWithDefault u ud M.empty (Just y)
rhs = M.applyWithDefault M.empty (Just (_ $ y)) u ud
in lhs == rhs