[Haskell-cafe] Word rigid in "`a' is a rigid type variable..."

[Thiago Negri]

>
> I think that more intuitive/understandable would be something like
>
>     'b' has too rigid type for 'a' ...
>
> At least, that is what I have to tell myself when I encounter this issue
>

I don't think this is quite correct.

As I'm a daily Java programmer, one thing that really troubled me was to
think as `a' being something like `Object', but it is wrong.

The `a' really means that the client of the function can define the type it
wants, and be precise.
I guess it's easier to see this in the value (result) of a function: "f ::
a -> b" is a function that takes any value and produces any value the user
wants, i.e. I can take an Int out of it, or a Double, or a String, or a
Foo, or a Bar. That's a huge difference between Java's "Object f(Object a)"
(a better comparison would be with "<A, B> B f(A a);", I guess).

I'm diverging a bit, but what I want to say is that there is no way to tell
that the type of "b" is "too rigid" for the type `a' , because the `a' can
be anything, even the exact type of "b".
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