Hi Paul,
maybe False (\x -> True) (find isTypeB ts)
This can be more neatly expressed as:
isJust (find isTypeB ts)
But your entire thing can be expressed as:
containsTypeB ts = any isTypeB ts
I recommend reading through the Prelude interface and the List
interface, it has many useful functions that will help.
Thanks
Neil
________________________________
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Paul Keir
Sent: 12 November 2008 10:09 am
To: haskell-cafe@haskell.org
Subject: [Haskell-cafe] Searching for ADT patterns with elem and
find
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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