I'm currently studying the use of overlapping instances, and I was hoping to instrument GHC to produce some variety of list of instances that overlapped. I haven't done any GHC hacking so far, so I'm not entirely familiar with the code base. Does anyone have any guidance on which modules I should likely need to modify?
None?-) Loading this {-# LANGUAGE FlexibleInstances #-} class C a instance C a instance C [a] instance C [()] instance C [Bool] instance C Bool into ghci 6.8.3, we can play with various types and get ghci to list overlaps that might match those types. Of course, there are some oddities. Also, the behaviour will differ slightly if we actually enable OverlappingInstances. But it should get you started. Claus *Main> (undefined :: C a => a) *** Exception: Prelude.undefined *Main> (undefined :: C a => a) :: b <interactive>:1:1: Overlapping instances for C b arising from instantiating a type signature at <interactive>:1:1-21 Matching instances: instance C a -- Defined at C:/Documents and Settings/cr3/Desktop/Overlap.hs:3:0-11 instance C Bool -- Defined at C:/Documents and Settings/cr3/Desktop/Overlap.hs:7:0-14 instance C [Bool] -- Defined at C:/Documents and Settings/cr3/Desktop/Overlap.hs:6:0-16 instance C [()] -- Defined at C:/Documents and Settings/cr3/Desktop/Overlap.hs:5:0-14 instance C [a] -- Defined at C:/Documents and Settings/cr3/Desktop/Overlap.hs:4:0-13 (The choice depends on the instantiation of `b' To pick the first instance above, use -fallow-incoherent-instances when compiling the other instance declarations) In the expression: (undefined :: (C a) => a) :: b In the definition of `it': it = (undefined :: (C a) => a) :: b *Main> (undefined :: C a => a) :: Bool <interactive>:1:1: Overlapping instances for C Bool arising from instantiating a type signature at <interactive>:1:1-21 Matching instances: instance C a -- Defined at C:/Documents and Settings/cr3/Desktop/Overlap.hs:3:0-11 instance C Bool -- Defined at C:/Documents and Settings/cr3/Desktop/Overlap.hs:7:0-14 In the expression: (undefined :: (C a) => a) :: Bool In the definition of `it': it = (undefined :: (C a) => a) :: Bool *Main> (undefined :: C a => a) :: [b] <interactive>:1:1: Overlapping instances for C [b] arising from instantiating a type signature at <interactive>:1:1-21 Matching instances: instance C a -- Defined at C:/Documents and Settings/cr3/Desktop/Overlap.hs:3:0-11 instance C [a] -- Defined at C:/Documents and Settings/cr3/Desktop/Overlap.hs:4:0-13 instance C [Bool] -- Defined at C:/Documents and Settings/cr3/Desktop/Overlap.hs:6:0-16 instance C [()] -- Defined at C:/Documents and Settings/cr3/Desktop/Overlap.hs:5:0-14 In the expression: (undefined :: (C a) => a) :: [b] In the definition of `it': it = (undefined :: (C a) => a) :: [b] Claus