[Haskell-cafe] Happy Parser problem

[Aaron Gray]

On 31 December 2010 13:21, Aaron Gray <aaronngray.lists at gmail.com> wrote:

> I am trying to get a grammar where keywords are also valid identifiers.
>
>
Sorry working now !

Aaron


> Been messing round with the following Happy grammar :-
>
> %token
>       'let'           { TokenIdent "let" }
>       'in'            { TokenIdent "in" }
>       ident           { TokenIdent $$ }
>       int             { TokenInt $$ }
>       '='             { TokenEq }
>       '+'             { TokenPlus }
>       '-'             { TokenMinus }
>       '*'             { TokenTimes }
>      '/'             { TokenDiv }
>       '('             { TokenOB }
>       ')'             { TokenCB }
>
> %%
>
> Exp   : 'let' Var '=' Exp 'in' Exp  { Let $2 $4 $6 }
>       | Exp1                    { Exp1 $1 }
>
> Exp1  : Exp1 '+' Term           { Plus $1 $3 }
>       | Exp1 '-' Term           { Minus $1 $3 }
>       | Term                    { Term $1 }
>
> Term  : Term '*' Factor         { Times $1 $3 }
>       | Term '/' Factor         { Div $1 $3 }
>       | Factor                  { Factor $1 }
>
> Factor :: { Factor }
>       : int                     { Int $1 }
>       | ident                   { Var $1 }
>       | '(' Exp ')'             { Brack $2 }
>
> Var :: { Factor }
>       : ident                   { Var $1 }
>       | 'let'                   { Var "let" }
>
> Here 'Var' should be able to represent a 'let' identifier, as well as a
> keyword. Happy accepts the grammar but it does not parser the expected 'let
> let = x in let'.
>
> I have attached the full Happy grammar.
>
> I don't think this is an LR thing, but could be wrong.
>
> Many thanks in advance,
>
> Aaron
>
>
>
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