New subject: [Diffusion] [Committed] rCABAL2863a628f857: Add two local type signatures

8 Jul 2016

      I don't believe I made this commit, or the ones around it.   Mysterious; I hope I'm not being impersonated!

Simon

From: noreply@phabricator.haskell.org [mailto:noreply@phabricator.haskell.org]
Sent: 08 July 2016 20:43
To: Simon Peyton Jones 
Subject: [Diffusion] [Committed] rCABAL2863a628f857: Add two local type signatures

simonpj committed rCABAL2863a628f857: Add two local type signatures (authored by simonpj).

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?!

AFFECTED FILES
/Distribution/ParseUtils.hs

USERS
simonpj (Author)

COMMIT
https://phabricator.haskell.org/rCABAL2863a628f857

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To: simonpj

RE: [Diffusion] [Committed] rCABAL2863a628f857: Add two local type signatures

Simon Peyton Jones

Austin Seipp

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participants (2)