This is what I "understand" so far ...
Suppose we have these two values:
a) \x->x + x
b) \x->2 * x
Because these to values are equal, all functions definable in Haskell must
preserve this.
This is why I am not allowed to define a function like
h :: (a->b) -> (a->b)
h x = x
The reasons are very complicated, but it goes something like this:
- when one put \x->x+x trough the function h, the compiler might change it
to \x -> 2*x
- when one put \x->2*x trough the function h, the compiler might change it
to \x -> x + x
And we all know that \x -> 2*x is not the same as \x->x+x and this is the
reason one cannot define h in Haskell
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