Question

I’m still new and trying to create a list for use in a function and want to keep it as small as possible which happens to be logBase x y.
but I’m having trouble getting logBase into something I can use in this list.

[1 .. (logBase x y)]

Any suggestions?

Answer 1

You don’t post what type error you get, but I imagine it is something like this:

Prelude> let x = 2
Prelude> let y = 7
Prelude> [1 .. (logBase x y)] 

<interactive>:1:7:
    No instance for (Floating Integer)
      arising from a use of `logBase' at <interactive>:1:7-17
    Possible fix: add an instance declaration for (Floating Integer)
    In the expression: (logBase x y)
    In the expression: [1 .. (logBase x y)]
    In the definition of `it': it = [1 .. (logBase x y)]

The problem is that:

Prelude> :t logBase
logBase :: (Floating a) => a -> a -> a

returns a type in the Floating class, while other variables in your program (1, ‘x’, ‘y’) are of integral type.

I presume you want a sequence of Integers?

Prelude> :set -XNoMonomorphismRestriction
Prelude> let x = 2
Prelude> let y = 42
Prelude> [1 .. truncate (logBase x y)] 
[1,2,3,4,5]

Use truncate, celing or floor.