Srinivas,
mkT :: (Typeable a, Typeable b) => (b -> b) -> a -> a
mkT f = case cast f of
Just g -> g
Nothing -> id
From the definition of cast and the examples given earlier it looks
like cast needs to be told what the target type (a here) is in
order to
know what its trying to cast to - hence the example was "(cast
'a')::Maybe Char" - which makes sense. So I dont quite understand the
usage of cast presented here inside mkT. How does the case proceed
without knowing what its trying to cast f to?
Well, it can proceed because all type information it really needs is
available.
Just consider:
The explicit signature reveals that mkT has type
(b -> b) -> a -> a
(for any a and b that are instances of Typeable).
Using this information we derive that f in
mkT f = ...
is a function of type b -> b, right?
Then the right-hand side: that one should give us a function of type
a -> a. Just skip the first part of the case statement for a while
and proceed to its body:
mkT f = case ... of
Just g -> g
...
Since, g has to have type a -> a and Just has type t -> Maybe t for
all t, it follows that Just g has type Maybe (a -> a). Therefore the
case statement should perform pattern matching on a value of type
Maybe (a -> a). So, from
mkT f = case cast f of
Just g -> g
...
it can be inferred that cast f has type Maybe (a -> a) and, hence,
that cast should try to case f to a function of type a -> a.
HTH,
Stefan
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