[Haskell-beginners] question on types

[Jake Penton]

On 2011-07-28, at 8:23 PM, Brandon Allbery wrote:

> On Thu, Jul 28, 2011 at 19:42, Jake Penton <djp at arqux.com> wrote:
> h:: a
> h = 'a'
> 
> to which ghci replies:
> 
>     Couldn't match type `a' with `Char'
>       `a' is a rigid type variable bound by
>           the type signature for c :: a
>           at /Users/David/Project/EoP/ch04/weak.hs:114:1
>     In the expression: 'a'
>     In an equation for `c': c = 'a'
> 
> This last example is probably the most basic one which I need to understand. But, why is the problem apparently a different one than in the definition of "f" above?
> 
> The other one was complicated by polymorphism:  a numeric literal is compiled into a call to fromIntegral, whose result type is Num a => a.
>  
> The problem is that, when you say something's type is "a", you are not saying "pick an appropriate type for me"; you are saying "whoever invokes this can ask for any type it wants" (equivalently:  "I promise to be able to produce *any possible* type").  But then you give the value as Num a => a in the first example and Char in the second example, neither of which is "any possible type".
> 
> An explicit type is a complete contract; having contracted to produce an "a" (any type), you can't then offer only a Char or a Num a => a.  You have to satisfy the contract which says "any type", otherwise you're doing the type checking equivalent of a bait-and-switch.
> 
> You can't express "pick a type for me" in a type signature; types are concrete, and a type variable in a signature is a concrete "anything", meaning the caller can request whatever it wants and you must produce it.  The type must be *completely* described by the signature; what it says is what you're committed to, and you can't then offer something else.  If you need a partial type signature, there are some tricks you can use which let you force types in the implementation without specifying a concrete signature (see http://okmij.org/ftp/Haskell/types.html#partial-sigs).
> 

Ok, thanks. I have some studying to do about haskell types, clearly.

Does that mean then that there is no definition possible of f other than 'undefined'? I mean this compiles:

f::a
f = undefined

But is there any other possible second line defining f?

- Jake -
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