How do you combine filter conditions

Question

The filter class of functions takes a condition (a -> Bool) and applies it when filtering.

What is the best way to use a filter on when you have multiple conditions?

Answer 1

Let's say your conditions are stored in a list called conditions. This list has the type [a -> Bool].

To apply all conditions to a value x, you can use map:

map ($ x) conditions

This applies each condition to x and returns a list of Bool. To reduce this list into a single boolean, True if all elements are True, and False otherwise, you can use the and function:

and $ map ($ x) conditions

Now you have a function that combines all conditions. Let's give it a name:

combined_condition x = and $ map ($ x) conditions

This function has the type a -> Bool, so we can use it in a call to filter:

filter combined_condition [1..10]