On Saturday 22 January 2011 23:11:07, aditya siram wrote:
The value of `null res' for a given String obviously depends on the type at which res is used.
Why does it matter what the type of res is as long as it is some kind of list? Doesn't 'reads' always gives [(a1,String)]? So why does it matter what type 'a1' is?
Because reads "True, that" :: [(Bool,String)] ~> [(True,", that")] and reads "True, that" :: [(Int,String)] ~> [] So to evaluate `null res', the type of res must be known/fixed. If it isn't, the compiler has two choices. It can refuse to compile or it can compile and let the runtime throw an error "Can't evaluate because I don't know the type" (I think the second option isn't available for GHC). In my opinion, the first option is preferable.
Thanks for the clear explanation of the monomorphism restriction. -deech