[Haskell-cafe] Efficiency Question

Derek Elkins ddarius at hotpop.com
Sat Jan 15 08:36:28 EST 2005

> Another thing, while toying, I found out that a comparison (n <= 0)
> takes three reductions more than (n < 1) according to my hugs, so
> changing the definition of splitAt thus, we require (3*n) reductions
> less. But the number of reductions and speed are different things, as
> witnessed by the above, so would it be an improvement to replace "n <=
> Integerliteral"-queries by "n < Integerliteral"-queries or doesn't
> that make any difference at all in execution time?

I don't know about in Hugs, but in (compiled, optimized) GHC it should
make no difference.  Presumably, if you are running something in an
interpreter micro-optimizing isn't worthwhile.  

> Finally, in several contexts I needed to cons an element to one of a
> pair of lists, so I defined
> infixr 5 &,§
> (&) :: a -> ([a],[b]) -> ([a],[b])
> x & (xs,ys) = (x:xs,ys)
> (§) :: b -> ([a],[b]) -> ([a],[b])
> y § (xs,ys) = (xs,y:ys).

Well, to start, the type signatures are unnecessarily restrictive. 
Then, the function that also is not in the Report, but does come up
quite a bit by people who get into a point-free or categorical style is
the bifunctor,
(***) :: (a -> b) -> (c -> d) -> (a,c) -> (b,d)
f *** g = \(a,b) -> (f a,g b)
this is an instance of (***) in Control.Arrow, hence the name.

So, your first function is,
(&) x = (x:) *** id
or using another function from Control.Arrow,
(&) x = first (x:)

I can say that I have wanted (***), I can't say that I've ever wanted
your two functions.  Also, first (x:) seems to be more self-documenting.

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