Creighton Hogg writes:

data Patootie = Pa Int | Tootie Int and I want to pull out the indices of all elements of a list that have type constructor Tootie, how would I do that?

x = [Pa 3, Tootie 5, Pa 7, Tootie 9, Pa 11] y = [ i | Tootie i <- x ] z = [ i | i@(Tootie _) <- x ] y or z might be helpful. -- Mark

On 10/02/06, Creighton Hogg wrote:

Hi, If I have something like data Patootie = Pa Int | Tootie Int and I want to pull out the indices of all elements of a list that have type constructor Tootie, how would I do that?

I thought I might be able to use findIndices, but I don't know how to express the predicate.

Just to clear up a small point, Tootie isn't a type constructor, but a data constructor. ('Maybe' is a type constructor, 'Just' is a data constructor.) You can use list comprehensions with pattern matching to write this fairly succinctly: tootieIndices xs = [i | (Tootie {}, i) <- zip xs [0..]] The {} matches whatever parameters Tootie might have, so this will continue to work even if you later extend the Tootie data constructor with more fields. If you want to match more carefully, you can of course put a variable there. - Cale