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.
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