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

[Brandon Allbery]

On Wed, Nov 13, 2013 at 11:37 AM, Vlatko Basic <vlatko.basic at gmail.com>wrote:

>     f :: a -> Bool
>     f a = let b = "x" in a == b
>
> compiler complains with
>   `a' is a rigid type variable bound by  the type signature for f :: a ->
> Bool
>
> I'm puzzled with the choice of word 'rigid' here.
> I see these types as
>     - 'b' has "rigid/unchangeable" type (only String), and
>     - 'a' has "soft/variable" type (any type, no constraints).
>

The type declaration is the final arbiter. Since it says "any type", it
means exactly that: you are claiming your function is prepared to handle
*any type* the caller wishes to specify. It is not "soft", nor "variable"
in the sense you intend: it is a hard requirement that your function must
be prepared to handle whatever type the caller wants there.

But instead your function requires that it be String, because both sides of
(==) must be the same type. This violates the type signature'a assertion
that the caller can specify any type.

-- 
brandon s allbery kf8nh                               sine nomine associates
allbery.b at gmail.com                                  ballbery at sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
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