alexteslin at yahoo.co.uk
Wed Jul 25 14:06:13 EDT 2007
Thank you Dan
Dan Weston wrote:
> If you find it tedious to pass parameters that never change, remember
> that you can access symbols defined in the enclosing environment
> (closure), freeing you from "passing it on" each time:
> filterAlpha :: (a -> Bool) -> [a] -> [a]
> filterAlpha f = filterAlpha'
> where filterAlpha'  = 
> filterAlpha' (x:xs)
> | f x = x : (filterAlpha' xs)
> | otherwise = (filterAlpha' xs)
> As far as the primed version filterAlpha' is concerned, f is a global
> symbol. The argument list is just used for values that vary.
> This will be your friend if the parameter list starts to stack up with
> more and more "reference" or "environment" inputs. It is also easier to
> see that f never changes if it is defined in only one spot.
> Later, when you study monads you will notice the same pattern in the
> Reader monad, where the technique is even more valuable.
> Dan Weston
> Alexteslin wrote:
>> filterAlpha :: (a -> Bool) -> [a] -> [a]
>> filterAlpha f  = 
>> filterAlpha f (x:xs)
>> |f x = x : filterAlpha f xs -- corrected
>> |otherwise = filterAlpha f xs -- corrected
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