This post follows on from a discussion about a month ago, called "Haskell Syntax Inside Quasiquote".
To summarise, suppose I want to create a Haskell quasiquoter for lists, eg [$list|1,x^2,y,3|] (representing the list [1,x^2,y,3]) Ideally, this would allow arbitrary Haskell expressions for each element of the list (provided, of course, that the types are correct). To write this list quasiquoter, I would need to parse Haskell expressions. This is doable[1], but is necessarily buggy when infix operators are concerned. For instance, a user of the list quasiquoter may define two new operators, (+*) and (+^), and then write [$list|1+*2+^3|] Being able to correctly parse such an expression depends on known the two operator's fixities, which is currently impossible. The problem is that potentially non-local fixity declarations affect parsing. As far as I can see, the correct solution is to change the Template Haskell AST representation of fixity expressions from > data Exp = ... > | InfixE (Maybe Exp) Exp (Maybe Exp) to something like > type Op = Exp --- because TH doesn't distinguish operators from expressions > data Exp = ... > | InfixE [Exp] [Op] --- length [Exp] == 1 + length [Op], always > | LeftSection Exp Op > | RightSection Op Exp Apart from splitting of the sections, the important difference is that the expressions are parsed as a list of expressions separated by operators. So, my example above would be parsed as something like InfixE [parseExp "1", parseExp "2", parseExp "3"] ["+*", "+^"] Then, when Template Haskell performs the splice, it could correctly resolve the fixities. Of course, this would require a change to Template Haskell, so a second-best solution would be to forbid unparenthesised expressions in my quasiquoter. Then, parsing can proceed correctly without knowing the fixities. This would be easiest to do if haskell-src-exts changed its AST in a similar way to described above for Template Haskell. Any thoughts? Cheers, Reiner Pope [1] For instance, see haskell-src-meta, which uses haskell-src-exts as a parser, and then converts the AST into a form that Template Haskell recognises. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
