Question

Possible Duplicate:
Return a list of Integer values that is not within a range of numbers in an existing list:

I’m looking to take the following:

[Vacant, Engaged, Vacant, Vacant, Engaged, Vacant, Vacant, Vacant, Vacant] 

and return a list of only Vacant values from the above list.

(i.e. [Vacant, Vacant, Vacant, Vacant, Vacant, Vacant, Vacant]).

Is there any predefined haskell function to perform this operation?

(n.b. Vacant and Engaged are data types)

Answer 1

It’s called filter:

filter (== Vacant)

Alternatively, use a list comprehension:

[x | x <- lst, x == Vacant]

(Slightly longer in this case, but more convenient in some other circumstances.)