Empty instance definitions don't parse

[NeilMitchell]

Parse error on:

instance Traversable Tree where

x :: Int
x = 4

Originally reported as a HLint bug at http://code.google.com/p/ndmitchell/issues/detail?id=156

[nicolas.pouillard@…]

Removing the 'where' keyword is an easy workaround, right?

[nibro]

Indeed, haskell-src-exts would handle an empty instance or class declaration that doesn't end in 'where'. But as adding the where is apparently allowed (and reportedly even means a slightly different thing semantically) then haskell-src-exts should naturally be able to parse it correctly.

It's a bit messy though, since it means a special case that the AST must record whether the where was given or not.

[nibro]

As an update, I've looked into this but I don't immediately realize what goes wrong. If the empty instance declaration is the last thing in the document, the parse succeeds. So this seems to be something of a layout problem, which will be a bit tricky to fix. I'll keep digging.

[nibro]

By far the trickiest bug so far. There was an error in the old haskell-src code that failed to insert a virtual close brace when the new layout context was not to the right of the parent context (meaning the new context had to be empty). But it's fixed now! Case closed.