Hi All, One important property of type class dictionary translation is coherence which basically says that two typing derivations for the same term at the same type in the same environment must be equivalent. This definition is established with the assumption of non-overlapping. In the GHC documentation which describes the extension of overlapping instances, an example similar to the following is given.
class C a where f:a -> a instance C Int where f=e1 instance C a where f=e2
let g x = f x in g 1
In this case GHC takes an ¡°incoherent¡± decision by taking the second instance as an instantiation of function f even it is executed with an input of type Int. My question is whether the ¡°incoherence¡± behaviour of overlapping instances is derived from the definition of coherence in the non-overlapping setting? If yes, how is it applicable to rule out the incoherent behaviour? If otherwise, what is the definition of coherence with overlapping instances? Thanks. --william _________________________________________________________________ Find just what you are after with the more precise, more powerful new MSN Search. http://search.msn.com.sg/ Try it now.