[Haskell-cafe] unboxed arrays restricted to simple types (Int, Float, ..)

Tom Nielsen tanielsen at gmail.com
Wed Nov 11 07:49:50 EST 2009

There's a couple of things going on here:

-If you use storablevector and storable-tuple, or uvector, you can
store tuples of things. So your stupidArrayElement could be mimicked
by (Int, Int).

-But what you want to do is store a variable-sized data type. How
would you do that in C? If you can spare another bit of memory, it
might be better to define type T = (Bool, Bitmask) and use
storablevector. Or maybe you want a sparse array of Bitmasks?

-Yes it is a shame that Haskell does not have good support for
unbounded polymorphic arrays. What if I want an array of functions?
Here's a little trick that can make it a bit easier to store any data
type in an unboxed array. I don't know, for instance, of any other way
to define unrestricted functor/applicative for unboxed arrays. This
trick should work with any other array library.


module FArray where

import Data.StorableVector
import Foreign.Storable
import Control.Applicative

data EqOrF a b where
    Eq :: EqOrF a a
    F :: (a->b) -> EqOrF a b

data FArray a where
    FArray :: Storable a => Vector a -> EqOrF a b -> FArray b
    ConstArr :: a -> FArray a

instance Functor FArray where
    fmap f (ConstArr x) = ConstArr $ f x
    fmap f (FArray sv Eq) = FArray sv $ F f
    fmap f (FArray sv (F g)) = FArray sv $ F $ f . g

instance Applicative FArray where
    pure x = ConstArr x
    (ConstArr f) <*> farr = fmap f farr
    -- other cases left as an exercise. Which is to say, my bladder is
bursting and I also need lunch.

arrayOfInts = FArray (pack [1..10]) Eq
arrayOfAdders = (+) `fmap` arrayOfInts


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