On Tue, Apr 10, 2007 at 02:09:03PM +0100, Joel Reymont wrote:
Folks,
Imagine a language where Num + Num yields a Num and Str + Num yields
a Str but Num + Str should not be allowed.
I implemented parsing for such a language in OCaml with a yacc-based
parser without an additional type-checking pass, entirely within the
yacc grammar. I tried to take such an approach with Parsec but hit
the roadblock with buildExpressionParser since it returns the same
type as the token parser given to it.
Just because you have a hammer doesn't mean you have a nail. I've
gotten along all this time never learning buildExpressionParser,
instead hand-coding my precedence parsers.
Rather than have numerical expressions, string expressions, etc., as
separate types, I simplified things down to a single expression type
that holds my booleans, strings and numbers. I now need to implement
type checking of my parsed AST.
Would something like this work for you:
data NumExpr = ...
data StrExpr = ...
data Expr = NumExpr NumExpr | StrExpr StrExpr
addExpr :: Expr - Expr - CharParser st Expr
addExpr (NumExpr n) (NumExpr m) = NumExpr (Add n m)
addExpr (StrExpr n) (NumExpr m) = StrExpr (Cat n (ToString m))
...
op3 ch fn a b = join $ liftM3 (\x _ y - fn x y) a (char ch) b
...
exp0 = op3 '+' addExpr exp1 exp0 ...
It might also be good to modify chainl1/chainr1, with support for
possibly failing operations.
Remember - the provided abstractions have failed you, but you can
still define your own!
The main issue is error reporting. I'm not sure where to get token
location with Parsec and how to elegantly embed it in my AST.
Has anyone implemented type checking on top of a Parsec-based parser?
How did you locate your tokens?
There's haskell-src:
http://haskell.org/ghc/dist/current/docs/libraries/haskell-src/Language-Haskell-Syntax.html
And people wonder why I decided to abuse template haskell instead of
using the standard AST type. You do NOT want to go there. Not until
Oleg invents a way to make this stuff automatic, anyway.
Stefan
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