DataKinds and Nat singletons

[acowley]

If we have {-# LANGUAGE DataKinds #-} we might use Nat literals in types.

foo :: V 8 a
foo = bar

The type here gets a reddish highlight in emacs for me, and gives me a sum-node-start: Wrong type argument: markers, nil message. Similar code in a much larger whole gives shm a lot of trouble.

[chrisdone]

Yeah, running M-x shm/test-exe yields: Parse error: 8 HSE doesn't know how to parse this extension yet: haskell-suite/haskell-src-exts#19 The proper fix is to update HSE. That's the bad news. The good news is there's a pull request for DataKinds support: haskell-suite/haskell-src-exts#58 So you could try installing that and then re-build structured-haskell-mode with the smarter parser.

I did in the early phases try to use the GHC parser, but the AST in there is full of _|_ traps that makes it very difficult to safely traverse generically.