Data.Typeable: types which should compare equal don't

[NickSmallbone]

Here are two ways to construct a TypeRep? for the type () -> ():

Prelude> import Data.Typeable
Prelude Data.Typeable> let unitToUnit = typeOf (\() -> ())
Prelude Data.Typeable> let unitToUnit' = mkFunTy (typeOf ()) (typeOf ())

It seems to work:

Prelude Data.Typeable> unitToUnit
() -> ()
Prelude Data.Typeable> unitToUnit'
() -> ()

But the two TypeReps? are not equal:

Prelude Data.Typeable> unitToUnit == unitToUnit'
False

[simonpj]

I have a fix for this coming.

[simonpj]

Excellent point thank you. Fixed by

commit f35ebbd5dfd108487efa7912349e9802f6029897
Author: Simon Peyton Jones <simonpj@microsoft.com>
Date:   Fri Mar 30 12:51:14 2012 +0100

    Fix an egregious bug in the fingerprint calculation for TypeRep
    
    Given (T ty1) and ty2, we were computing the fingerprint of the
    application (T ty1 ty2) by combining the two fingerprints from (T ty1)
    and ty2.  But that gives a different answer to combinging the three
    fingerprints from T, ty1, and ty2, which is what happens if you
    build the type all at once.  Urk!
    
    Fixes Trac #5962

 Data/Typeable/Internal.hs |    9 ++++++---
 1 files changed, 6 insertions(+), 3 deletions(-)

[simonmar]

Aha, good bug!

[simonmar]

We really should have merged this.

[pcapriotti]

Merged as dd2af98d191d762e68e0b4c916096afad8b04dd7.