symbol includes trailing @ sign in as-patterns

[mattraibert]

Emacs 24.3.91.1 (built from src on ubuntu)
haskell-mode 20140726.1852 (from melpa)

When I use find-tag or highlight-symbol-at-point on an as-pattern, emacs incorrectly thinks the at-sign is part of the symbol.

For example, in the following code, if point is on the xxs on the left-hand side of line 3, and you type M-. to find-tag, the default symbol to search for will be xxs@

tails                   :: [a] -> [[a]]
tails []                =  [[]]
tails xxs@(_:xs)        =  xxs : tails xs

It's also observable in this screenshot of this code store@ is highlighted but not store

[purcell]

Yes, this is a tricky one. @ is explicitly added to the symbol table as a "symbol constituent" character, since it is indeed valid in certain positions:

    (mapc (lambda (x)
            (modify-syntax-entry x "_" table))
          ;; Some of these are actually OK by default.
          "!#$%&*+./:<=>?@^|~")

This discussion has a succinct summary of when it is legal, ie. not at the end of a symbol.

Given the Emacs syntax table system, which can't take into account the character's position in a potential symbol, I'm not sure that a resolution of this issue is possible. It might be the case that the syntax-propertize-function mechanism in Emacs >= 24 would help, but haskell-mode doesn't use that at all currently.

[gracjan]

@purcell: wait, this explanation does not sound right, please correct me if I'm wrong:

There is a 'w' category that is for words. '@' is never a part of 'w' category in Haskell.
There is a '_' category that is for symbols. '@' is either special symbol if alone or part of operator symbol if there is a longer sequence of symbols.

Am I wrong about either Haskell or Emacs interpretation of 'w' vs '_'?

[purcell]

@gracjan Hmm, yes, you're probably right that @ should not have symbol syntax.

[gracjan]

@purcell, @mattraibert

Currently '@' has class '_', each of 'store' has class 'w'.

In haskell-mode highlight-symbol-at-point highlights whole of 'store@', and whole of 'store+++' and whole of 'store+load'.

In c-mode characters like '+', '@' etc have class '.' (punctuation).

Do we lose any functionality if Haskell symbol characters get puntuaction class?

[purcell]

Do we lose any functionality if Haskell symbol characters get puntuaction class?

Probably not, though I can't be 100% certain.