Re: [Haskell-cafe] Parser problem continued
expr :: Parser Int expr = do t - term do symbol + e - expr return e return (t + e) +++ return t- 't' is not in scope at the arrow. t only exists inside the do block, and your code parses like this ( do t - return (t+e) ) +++ ( return t ) perhaps like this: expr = do t - term (do symbol + e - expr return (t+e) ) +++ (return t) although I think you may also want a 'try' before the first alternative. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] Parser problem continued
perhaps like this: expr = do t - term (do symbol + e - expr return (t+e) ) +++ (return t) although I think you may also want a 'try' before the first alternative. No, that still gives the same undefined variable error. :( Nik Dr Nik Freydís Whitehead University of Akureyri, Iceland * Having the moral high ground is good. Having the moral high ground and an FGMP-15 is better. * ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Parser problem continued
On Tue, Mar 15, 2005 at 03:44:55PM -, Nicola Whitehead wrote: perhaps like this: expr = do t - term (do symbol + e - expr return (t+e) ) +++ (return t) although I think you may also want a 'try' before the first alternative. No, that still gives the same undefined variable error. :( Sometimes I prefer to write such code in a lispy way: expr = do t - term ((+++) (do symbol + e - expr return (t+e)) (return t)) Best regards Tomasz ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Parser problem continued
The layout of your code is very important when writing haskell code:
Your code :
expr = do t - term
do symbol +
e - expr
return e
return (t + e)
+++ return t
is equivalent to:
expr = do { t - term
; do { symbol +
; e - expr
; return e
}
; return (t + e) -- e is not in scope
}
+++ return t -- t is not in scope
Both t and e are not in scope:
* e is in a nested do-block
* the expression 'return t' is outside the do-block
What you probably mean is:
expr = do t - term
do symbol +
e - expr
return (t + e)
+++ return t
which is equivalent to:
expr = do { t - term
; do { symbol +
; e - expr -- (dropped the return e line)
; return (t + e)
}
+++ return t
}
Now t and e are in scope. The parser 'expr' will first recognize a
'term' and then try to recognize a '+' symbol followed by an
expression. If that fails it returns 't'.
Cheers,
Arthur
On 15-mrt-05, at 16:28, Nicola Whitehead wrote:
Curiouser and curiouser...
expr :: Parser Int
expr = do t - term
do symbol +
e - expr
return (t + e)
+++ return t
solves the undefined variable problem but introduces a new 'Last
operator in do {...} must be an expression' error, which then
disappears if I explicitly return e
expr :: Parser Int
expr = do t - term
do symbol +
e - expr
return e
return (t + e)
+++ return t
to give me the original undefined variable t error at the line expr =
do t - term . It looks in scope to me... :(
Thanks,
Nik
Dr Nik Freydís Whitehead
University of Akureyri, Iceland
*
Having the moral high ground is good.
Having the moral high ground and an FGMP-15 is better.
*
___
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RE: [Haskell-cafe] Parser problem continued
Thanks folks! Writing it in a lispy manner seems to work. I see what Arthur means about the layout - I think I'm still thinking too much in C. :) Nik Dr Nik Freydís Whitehead University of Akureyri, Iceland * Having the moral high ground is good. Having the moral high ground and an FGMP-15 is better. * ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
