External Core needs love

[quux]

{-# LANGUAGE GADTs #-}

data T a b where
  T1 :: a -> b -> T [a] (a,b)
  
main = putStrLn ":("

and then "ghc this.hs -fext-core" results in

>ghc core-sandbox.hs -fext-core
ghc core-sandbox.hs -fext-core
[1 of 1] Compiling Main             ( core-sandbox.hs, core-sandbox.o )
ghc: panic! (the 'impossible' happened)
  (GHC version 7.2.1 for i386-unknown-mingw32):
	make_exp (App _ (Coercion _))

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

[simonpj]

External Core is defined by a data type in coreSyn/ExternalCore.lhs. This data type has received No Love for years. As a result, it is lagging way behind Core, as is External Core's concrete syntax. For example it does not accommodate GADTs at all, I think. In this particular ticket, we are trying to convert a Coercion from Core to ExtenalCore.Exp, but there are no coercions (yet) in Exp.

In short, External Core needs some love and attention.

(I suspect it's ended up in this state because most people are using the GHC API instead, but the idea of giving Core a proper, printable external realisation remains a good one..)

If anyone feels able to draft it into the 21st century I'd be happy to advise. For what it's worth, I think that a promising approach might be to abolish the ExternalCore data types in favour of those in IfaceSyn, which serve a very similar purpose. The IfaceSyn types didn't exist when External Core was first implemented, but they very much do now, and they *cannot* lag.

So I suggest that we abandon ExtCore as it is, and instead write a printer and parser for IfaceSyn. But it needs a volunteer.

Simon

[tim]

The reason for having the ExternalCore data types is that External Core has a semantics (through the ext-core typechecker and interpreter), and I'm not sure whether IfaceSyn? does (though I'm not really familiar with it). (Of course, the semantics probably doesn't reflect GADTs right now...)

I'm not really sure I'm up for giving ext-core more love at this point, but I wanted to recommend some caution...

Tim