One difficulty is that many of the core type data types, e.g. TyThing, are (1) a large mutually recursive graph, and (2) have unsafeInterleaveIO thunks which would induce IO action. So a naive Show instance would give infinite output and have lots of side effects. There are many data types which could usefully have Show added but also many which would be very difficult to.
Ah, yes, this is a very annoying problem. I discovered that when I first wrote
CoreDump: https://github.com/osa1/CoreDump/issues/2
I don't have solution to this yet.
2015-10-19 21:59 GMT-04:00 Edward Z. Yang
Excerpts from Ömer Sinan Ağacan's message of 2015-10-19 14:18:41 -0700:
I was wondering what would be the cost of adding Show instances. Would that mean significantly increased compile times? Or significantly bigger GHC binaries? If that's the case, could we enable Show instances with some arguments so that we can enable/disable it by modifying mk/build.mk?
One difficulty is that many of the core type data types, e.g. TyThing, are (1) a large mutually recursive graph, and (2) have unsafeInterleaveIO thunks which would induce IO action. So a naive Show instance would give infinite output and have lots of side effects. There are many data types which could usefully have Show added but also many which would be very difficult to.
Edward