[Haskell-cafe] F# active patterns versus GHC's view

[John Van Enk]

2009/1/16 Peter Verswyvelen <bugfact at gmail.com>

> [...]
>
> After a while you decide that you need to change the Bla data type, maybe
> give Dog more fields, maybe completely redesign it, maybe not exposing it,
> but you want to keep existing code backwards compatible. With F# you can
> write Active Patterns for the old Dog and Cat constructors, so all existing
> code remains compatible. At least that is the way I understand it, but I
> have not actually worked yet with Active Patterns, will do so soon :)
>
>
You get something similar with the record syntax (though, probably still not
quite as powerful as the active patterns):

> data Blah = Dog { dogA :: Int }
>           | Cat { catA :: Int }

> f :: Blah -> Int
> f (Dog {dogA = a}) = a
> f (Cat {catA = a}) = a

f (Dog { dogA = 1})
  ==> 1
f (Cat { catA = 1})
  ==> 1

If we add more fields:

> data Blah = Dog { dogA :: Int , dogB :: Int }
>           | Cat { catA :: Int , catB :: Int }

> f :: Blah -> Int
> f (Dog {dogA = a}) = a
> f (Cat {catA = a}) = a

f (Dog {dogA = 1, dogB = 2})
  ==> 1
f (Cat {catA = 1, catB = 2})
  ==> 1

Note that the definition of 'f' doesn't have to change. If you decide to
remove that specific field, then you'll have to refactor f, but the act of
adding *more* fields doesn't impact the existing definitions using record
pattern matching.

The addition of record disambiguation and wildcards[1] cleans this up a bit
more.

> {-# OPTIONS -XRecordWildCards #-}
> data Blah = Dog { a :: Int , b :: Int , c :: Int }
>           | Cat { a :: Int , b :: Int }

> dog = Dog {a = 1, b = 2, c = 12}
> cat = Cat {a = 1, b = 2}

> f :: Blah -> Int
> f (Dog {..}) = a
> f (Cat {..}) = a

f dog
  ==> 1
f cat
  ==> 1

So, it still might not be able to pull everything off, but it sure covers
most of the bases. The cards are especially useful.

I <3 Haskell.

/jve

[1]:
http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.haskell.org/pipermail/haskell-cafe/attachments/20090116/6a590e32/attachment.htm