[Haskell-beginners] question on types

[Julian Porter]

The point is that in

f::a

you are saying that f is of an undefined type a.  But in 

f=1

you're saying that a must be a type that supports the value 1, and not all do (eg strings, booleans).

Therefore you have to tell the compiler that you are constraining a to allow it to take the value 1, hence the constraint Num a, which does just that.

The error message is telling you just this: it can't find a constraint that tells it that a is in the class Num, so it is complaining because what you have written could fail if you used f in a context where it was inferred to have a non numerical type.  Constraints allow the type-inference engine to spot such errors, saving you from terrible grief in debugging.

Sent from my iPhone

On 29 Jul 2011, at 00:42, Jake Penton <djp at arqux.com> wrote:

> Yikes.
> 
> I have been doing a fair bit of productive programming in Haskell, thinking that I am making a bit of progress. Then I hit something that is apparently *really simple* that I do not understand at all. How discouraging.
> 
> Here is the code that makes me realize I don't understand types or type inference very well at all yet:
> 
> f :: a
> f = 1
> 
> When I try to load the above, ghci gives me:
> 
>     No instance for (Num a)
>       arising from the literal `2'
>     In the expression: 2
>     In an equation for `f': f = 2
> Failed, modules loaded: none.
> 
> 
> Ok, so then I try:
> 
> g:: (Num a) => a
> g = 2
> 
> This compiles.
> 
> Why? I mean, why is the first example (defining f) wrong, but the second example (defining g) ok?
> 
> A slight variation on this is:
> 
> 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?
> 
> Of course, I cannot think of a reason to actually define things as shown above under ordinary circumstances. The code above is just boiled down to the simplest case I could find to illustrate my confusion.
> 
> I guess I interpret "f::a" to mean "f is some (any) type a". So why can't it be whatever "1" is, which I suppose is Integer. What is the type system looking for? And why does the constraint (Num a) make things ok?
> 
> Please point me in the direction of any reading I should do to clear up my confusion.
> 
> TIA.
> 
> - Jake -
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