RE: [Haskell-cafe] Searching for ADT patterns with elem and find

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.

Nope, it will stop on the first one - Haskell is lazy like that :-) Thanks, Neil ________________________________ From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Paul Keir Sent: 12 November 2008 1:45 pm To: Tom Nielsen Cc: haskell-cafe@haskell.org Subject: RE: [Haskell-cafe] Searching for ADT patterns with elem and find 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 > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > ============================================================================== Please access the attached hyperlink for an important electronic communications disclaimer: http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html ==============================================================================