Searching for ADT patterns with elem and find

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: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] 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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