Thanks Tom,
That is indeed a very elegant solution; I too often forget about the wonders of list comprehension.
I guess one drawback compared to Neil's suggested use of "any" (and staying with a separate "isTypeB") is that your solution will iterate over the entire list, regardless of an early hit.
But I don't think your second (as-pattern) solution for findBs is ugly; I quite like it actually.
Cheers,
Paul
-----Original Message-----
From: Tom Nielsen [mailto:tanielsen@gmail.com]
Sent: Wed 12/11/2008 12:39
To: Paul Keir
Cc: haskell-cafe@haskell.org
Subject: Re: [Haskell-cafe] Searching for ADT patterns with elem and find
somebody pointed out a few months back that list comprehensions do this nicely:
containsTypeB ts = not $ null [x | (B x) <- ts]
no need for defining isTypeB.
not quite sure how you would write findBs :: [T]->[T] succinctly; maybe
findBs ts = [b | b@(B _) <- ts]
or
findBs ts = [B x | (B x) <- ts]
both of them compile but the first is ugly and the second is
inefficient (Tags a new T for every hit).
Tom
2008/11/12 Paul Keir
Hi All,
If I have an ADT, say
data T = A String Integer | B Double | C deriving(Eq)
and I want to find if a list (ts) of type T contains an element of subtype "B Double", must my "containsTypeX" function use a second "isTypeX" function as follows:
isTypeB :: T -> Bool isTypeB (B _) = True isTypeB _ = False
containsTypeB :: [T] -> Bool containsTypeB ts = maybe False (\x -> True) (find isTypeB ts)
I understand that while something like "find C ts" will work, "find (isTypeB _) ts" will not, but is there no such thing as a pattern combinator(?), or lambda that could help with this situation. I find I have many individual "isTypeB" functions now.
Regards, Paul
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