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

[Ian Lynagh]

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