[Haskell-cafe] Inline makes program slow?

[Tom Ellis]

On Thu, Apr 03, 2014 at 04:21:38PM +0000, Simon Peyton Jones wrote:
> Dominick's program is very delicate:
> 
> 	fibonacci :: (Integral a) => [a]
> 	fibonacci = 0 : 1 : zipWith (+) fibonacci (tail fibonacci)
> 
> Remember, the "(Integral a) =>" thing is very like "Integral a ->"; it's
> an extra value argument.  Would you expect this program to be fast?
> 
> 	foo :: Int -> [Int]
> 	foo n = 0 : 1 : zipWith (+) (foo n) (tail (foo n))

Indeed.  The curious may be interested to know you can fix this by hand by
sharing a monomorphic bind

    fibonacci = let f = fibonacci in 0 : 1 : zipWith (+) f (tail f)

unless the dreaded monomorphism restriction is off, in which case you have
to find some other way of making 'f' monomorphic, for example with
ScopedTypeVariables:

    fibonacci = let f = fibonacci :: [a] in 0 : 1 : zipWith (+) f (tail f)

(I guess examples like this are exactly the reason the monomorphism
restriction exists).

Tom