pattern matching with functions in Haskell

Question

I've noticed the following peculiarity in Haskell:

 data Tree a = Empty  | Branch a (Tree a) (Tree a) deriving (Show, Eq)

leaf:: a->Tree a
leaf x = Branch x Empty Empty


findNumL:: Tree a->Integer
findNumL (Empty) = 0
findNumL (Branch x Empty Empty) = 1
findNumL (Branch x left right) = (findNumL left) + (findNumL right)

This is code will run perfectly fine, and it will return the number of leaves in a binary tree. However, if one tries to call the function leaf x instead of Branch x Empty Empty, the pattern recognition breaks down, making the definition of leaf x much less useful than it could be. Is there a way to circumvent this issue and use the leaf in pattern matching?

I cannot reproduce, probably I don't understand the question. This one works fine findNumL $ Branch "root" (leaf "left") (leaf "right") and so does this findNumL $ leaf "root" — Henri Menke
– Henri Menke, Commented Apr 2, 2018 at 0:11
@HenriMenke My apologies. I mean in the function definition, if I try findNumL (leaf x) = 1 in the second line of the function definition, the code will not compile. — Andres Mejia
– Andres Mejia, Commented Apr 2, 2018 at 0:13

Henri Menke · Accepted Answer · 2018-04-02 00:26:04Z

2

You can use the language extension PatternSynonyms

{-# LANGUAGE PatternSynonyms #-}

data Tree a = Empty  | Branch a (Tree a) (Tree a) deriving (Show, Eq)
pattern Leaf a = Branch a Empty Empty

findNumL:: Tree a->Integer
findNumL (Empty) = 0
findNumL (Leaf x) = 1
findNumL (Branch x left right) = (findNumL left) + (findNumL right)

Live example

answered Apr 2, 2018 at 0:26

Henri Menke

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9 Comments

Andres Mejia Over a year ago

does this still keep Leaf as a function a->Tree?

Henri Menke Over a year ago

@AndresMejia No, but you don't need it anymore. You can just use Leaf "x" instead of leaf "x".

Andres Mejia Over a year ago

I'm also still getting an error: "data constructor not in scope"

Henri Menke Over a year ago

@AndresMejia Does this happen with the exact example I showed on ideone? ideone.com/YAlxEq

Potato44 Over a year ago

@AndresMejia if you are using GHCI interactively rather than compiling with GHC you may need to :set -XPatternSynonyms

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