That is error-prone.
Plus the code does not need ScopedTypeVariables. The real problem comes
from the use of a class constraint on the LegGram data constructor.
data LegGram nt t s = Ord nt => LegGram (M.Map nt [RRule nt t s])
Short answer: you *can't *add class constraints to an already declared type
variable:
The LegGram *type *constructor declares the 'nt' variable (and then brings
it into scope), so trying afterwards to add a constraint to it for the
LegGram *data *constructor is invalid, so the compiler understands this:
data LegGram nt t s = *forall nt.* Ord nt => LegGram (M.Map *nt*
[RRule *nt*t s])
...the declaration and scoping of a *new* type variable.
This is it, right?
2012/1/3 Yucheng Zhang
As I understand it, both ways work.
legSome ((LegGram g)::LegGram nt t s) ntV
If you compile this without ScopedTypeVariables extension, GHC will remind you of it:
Illegal signature in pattern: LegGram nt t s Use -XScopedTypeVariables to permit it
So another solution is to avoid the ugly redundancy is:
legSome :: forall nt t s . LegGram nt t s -> nt -> Either String ([t], s)
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