[Haskell-cafe] A simple beginner question
dons at galois.com
Tue Jun 3 23:56:32 EDT 2008
> data SampleType = A | B Int | C String | D -- .... etc.
> sampleTypes = [A, B 5, C "test"] :: [SampleType]
> How do I find for example element A in the sampleTypes list?
> Do I have to create e.g.:
> isA :: SampleType -> Bool
> isA A = True
> isA _ = False
> for every constructor and use find?
> It feels like this is not the quicker method.
This is where the implicit filtering in a list comprehension comes in
data T = A | B Int | C String | D
xs = [A, B 5, C "test", D]
main = do print [ A | A <- xs ] -- filter A's
print [ x | x@(C _) <- xs ] -- filter C's
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