Haskell pattern matching symmetrical cases

Question

Suppose I have a haskell expression, for example:

foo (Nothing, Just a) = bar a foo (Just a, Nothing) = bar a 

Is there any haskell syntax to collapse these cases, so I can match any pattern and specify bar a as the answer for both? Or is it as brief as I can understand it?

Answer 1

It is as red as in Haskell. There is syntax in ML for what you want (by writing several patterns that bind the same variables, next to each other, separated by | body after the last pattern), but this is not the case in Haskell.

Answer 2

If your code is more complex than your example, you can do something like this using the Alternative instance for Maybe and the PatternGuards extension (part of Haskell2010).

 {-# LANGUAGE PatternGuards #-} import Control.Applicative foo (x, y) | Just a <- y <|> x = bar a 

If you are not familiar with it, <|> selects the leftmost Just , if any, and returns Nothing otherwise, which will cause the template to fail to protect.

Answer 3

You can use -XViewPatterns to add arbitrary functions to collapse your two cases into one template. Now your template is a p function that gives what you want to combine:

 foo (p -> (Just a, Nothing)) = bar a 

much easier!

We must define p , though:

 p (Nothing, a@(Just _)) = (a, Nothing) pa@(Just _, Nothing) = a pa = a 

or you want to normalize the data before viewing.

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Links: GHC Viewer Templates chapter