On Fri, Aug 16, 2013 at 12:49 AM, Bryan Vicknair <bryanvick@gmail.com> wrote:

I guess it is because the literal '5' could be an Int or a Float, and the type
system doesn't know.

The literal '5' is *known* to be of the *polymorphic* type (Num a => a). Ints and Floats are monomorphic instances of this polymorphic type. See

http://www.haskell.org/onlinereport/basic.html#numeric-literals

When I saw that the following does not work...::

bar :: (Show a) => a
bar = False

...it made a bit more sense. It seems that if a literal can be considered part
of a typeclass in more than one way, you can declare that literal to be of that
typeclass. However, if there is only one way for a literal to belong to a
typeclass, then you can't.

The part "if a literal can be considered part of a typeclass in more than one way" isn't how you want to think about them (see link). Good attempt though.

> I don't have a need for this, but now I'm curious: is there a way to declare
that all literal numbers in a module are of a certain concrete type? Something
like -XAllNumbersAreInt8?

You enforce monomorphism by specifying type signatures where appropriate. In some cases, a light touch is all that's needed because type inference does the rest.

-- Kim-Ee