On 28/07/2010 17:46, Simon Peyton Jones wrote:
Wed Jul 28 09:35:39 PDT 2010 simo...@microsoft/com
* Add two local type signatures
I'm adding these type signatures to satisfy the "do not generalise
local let/where" rule that GHC is taking on.
The signatures are clearly correct, but I was surprised at the
polymorphism needed. For example
parseOptVersion :: ReadP r Version
parseOptVersion = parseQuoted ver<++ ver
where ver :: ReadP r Version
ver = parse<++ return noVersion
noVersion = Version{ versionBranch=[], versionTags=[] }
Note that 'ver' really is called at two different types! That
in turn is because of the type of (<++)
(<++) :: ReadP a a -> ReadP r a -> ReadP r a
(+++) :: ReadP r a -> ReadP r a -> ReadP r a
Note the "a a" in the first arg, which is very unusual.
For example, compare the type of (+++).
Changing it to match the type of (+++) makes ReadP fail to compile,
though, so I assume it's right as it stands. But surely this deserves
a comment?!
Perhaps whoever wrote it didn't understand it well enough to write a
comment. That wouldn't surprise me, having grappled with a similar
situation using MonadCont recently. ReadP is a continuation monad, and
the first type argument is the "result" of the whole computation. In
the GHC implementation of ReadP this type argument is universally
quantified, but this module is using the "portable" version of ReadP
which exposes the type argument to avoid requiring RankNTypes.
Cheers,
Simon
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