Find Haskell Int precision

Question

Int in Haskell uses the machines native precision. I know I can query the bounds using maxBound and minBound. Is there a similar way of querying the precision width?

Something like

getPrecision :: Int -> Int

which returns say either 32 or 64.

Answer 1

One possibility is

import Data.Bits

getPrecision = popCount (maxBound :: Int) + 1

Answer 2

The cleanest way, for recent libraries (the ones that ship with GHC 7.8 or later), is to use finiteBitSize from Data.Bits . This is exactly the function you requested. With earlier versions, you can use bitSize , also from Data.Bits , a non-total version of the same thing. Specifically, bitSize will throw an exception if it's given something like Integer that doesn't have a bit size. finiteBitSize is in the FiniteBits subclass, so applying it to an Integer will give a type error. There's also a bitSizeMaybe in recent libraries, but that seems generally less useful.

Answer 3

You could use sizeOf but this comes with the caveat that the type must not have any padding bits.

import Foreign (Storable, sizeOf)

getPrecision :: Storable a => a -> Int
getPrecision dummy = sizeOf dummy * 8