PROPOSAL: Restrict the type of (^), (^^), and add genericPower, genericPower'

[Cale Gibbard]

On 17/11/2007, Ian Lynagh <igloo at earth.li> wrote:
>
> Hi all,
>
> This got a warm reception when I mentioned it in
>     http://www.haskell.org/pipermail/haskell-cafe/2007-June/027557.html
> so I'm formally proposing it now. It's trac #1902:
>     http://hackage.haskell.org/trac/ghc/ticket/1902
>
> Note that this is a divergence from Haskell 98 (but the libraries
> already have a handful of small divergences, and Haskell' is just around
> the corner...).
>
> In my opinion, (^) has the wrong type. Just as we have, for example,
>     (!!)         ::                 [a] -> Int -> a
>     genericIndex :: (Integral b) => [a] -> b   -> a
> we should also have
>     (^)          :: (Num a)             => a -> Int -> a
>     genericPower :: (Num a, Integral b) => a -> b   -> a
> (or some other function name). The same goes for (^^) (genericPower').
>
> In my experience this would remove 99.9% of all defaulting (mostly where
> you write things like x^12 and 8^12), which means it's easier to get
> -Wall clean without having to put :: Int annotations everywhere.
>
> The impact to GHC's bootlibs and extralibs is minimal. In most cases we
> have things like 2^15, where Int is clearly fine, although it happens to
> be defaulted to Integer currently. In Data.Complex we have 2 cases of
> e^(2::Int) which can now be beautified. There are several cases where
> the type is inferred to be Int anyway.
>
> There are 3 files where we really do have an Integer, and it does
> matter. They are all for parsing numbers of the form 18e43, in
> base/Text/Read/Lex.hs, parsec/Text/ParserCombinators/Parsec/Token.hs and
> haskell-src/Language/Haskell/Lexer.hs.
>
> Initial deadline: 1 Dec 2007.
>
>
> Thanks
> Ian

This is a move in the opposite direction from what I'd really like to
see. The Int type is usually a premature optimisation, and I usually
prefer to work with Integer as much as possible, but this just means
more fromIntegral conversions (or the use of the awkwardly named
general version).

I would much prefer for length, !!, etc. to have more general types,
not less general (with compiler specialisation on Int of course).

This change would be annoying.

 - Cale