CODEWITHSUNDEEP
haskellpattern-matching
ccQpein
ccQpein

Reputation: 815

Haskell pattern matching match Map.empty any with any Map.Map k v

I am writing a function which has different operations depending on Map.Map in argument if Map is empty or not

Here is my code:

import qualified Data.Map.Lazy as Map

testF :: (Ord a, Num a) => Map.Map a a -> [a] -> [a]
testF empty _  = [1024]
testF _ []     = []
testF m (x:xs) = [x] ++ (testF m xs)

main = do
  let a = Map.fromList [(1,2), (3,4)]
  print $ testF Map.empty [1,2,3] -- show [1024]
  print $ testF a [1,2,3] -- show [1024] too
  print $ a == Map.empty -- False

Of course GHC already give me notification that lastest line of function is redundant.

Pattern match is redundant
   In an equation for ‘testF’: testF m (x : xs) =
  1. Why every Map.Map can match Map.empty?
  2. What should I do to implement my requirement?

Thank you.

Upvotes: 1

Views: 1858

Answers (1)

melpomene
melpomene

Reputation: 85827

You're not matching against Map.empty, you're matching against empty, which is just a local variable bound to whatever map is coming in. You can't match against Map.empty because it's not a constructor.

What you can do instead is:

testF m _ | Map.null m = [1024]

I.e. use a guard to check whether m is empty.

Upvotes: 7

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