data Transl l g = Transl (l -> g) (g -> Maybe l) idT :: Transl b b
Constructors in Haskell are not first class values, that is, they can not be passed as argument to a function and then used in a pattern to match agains a value.
To be able to pass constructors as arguments to functions and use them both
to construct and destruct values, we use the type Transl. With a
a little help from cpp, we can easily construct Transl values
from constructors:
#define CON(c) (Transl (c) (\x->case x of c y->Just y;_->Nothing))
