On Saturday 01 January 2011 15:07:13, Thomas Davie wrote:
On 1 Jan 2011, at 12:38, Andreas Baldeau wrote:
So the question is, if ghc could transform this to simply compare the keys throwing away unsafePerformIO and return.
Wouldn't a much better plan simply be to take typeRepKey out of the IO monad?
I don't think so, it's in IO for a reason:
-- | Returns a unique integer associated with a 'TypeRep'. This can -- be used for making a mapping with TypeReps -- as the keys, for example. It is guaranteed that @t1 == t2@ -- if and only if @typeRepKey t1 == typeRepKey t2@. -- -- It is in the 'IO' monad because the actual value of the key may -- vary from run to run of the program. You should only rely on -- the equality property, not any actual key value. The relative -- ordering of keys has no meaning either. -- typeRepKey :: TypeRep -> IO Int
That also means that an Ord instance based on the keys may change from run to run. It's probably not a problem for applications if it's only used for storing TypeReps in a Map and not for programme logic (if typeOf 'a' < typeOf True then this else that), but it's somewhat fishy nevertheless.