[Haskell-cafe] Parser question

[Nicola Whitehead]

Hi 
folks,

I have a parser 
problem. I have a basic calculator program (Graham Hutton's from Nottingham) 
which contains the following code:

-- Define a parser 
to handle the inputexpr :: Parser Intexpr = do t - 
term 
do symbol 
"+" 
e - 
expr 
return (t + e) +++ 
return t
term :: Parser Intterm = do f - 
factor 
do symbol 
"*" 
e - 
expr 
return (f * t) +++ 
return f
factor :: Parser Intfactor = do symbol 
"(" 
e - 
expr 
symbol 
")" 
return e 
+++ natural
symbol and natural 
are defined elsewhere and work fine, but when I compile it I get the 
error

ERROR "C:/HUGS/Calculator.hs":66 - Undefined 
variable "t" 

I suspect I'm 
missing something obvious, but for the life of me I can't see it. Any 
suggestions?

Thanks,

Nik
(Trying to keep a 
couple of weeks ahead of her students)

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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[Haskell-cafe] Parser problem continued

[Nicola Whitehead]

Curiouser and 
curiouser...

expr :: 
Parser Intexpr = 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 Intexpr = 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

[Nicola Whitehead]

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.
*

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RE: [Haskell-cafe] Parser problem continued

[Nicola Whitehead]

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.
*

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