[Haskell-beginners] A few really short beginners questions

[Andrew Sackville-West]

On Sun, Oct 03, 2010 at 08:00:19PM +0200, Klaus Gy wrote:
> Hi! I have a few questions to improve my knowledge of Haskell. I
> didn't stumble over these problems while working on specific tasks, I
> more or less constructed them explicitly to get an adequate
> understanding of the basic Haskell semantics.

I'm fairly noobish at haskell, but I can answer some of them I think.

[...]
> 2
> 
> Why is the following example not valid?
> 
>   f :: a -> a
>   f '0' = 0
>   f x = 1
> 
> I think the reason lies in the type system but I can't locate it exactly.
> 

in the type declaration, you have claimed that f takes an object of
one type and returns another object of that same type. That is, every
instance of 'a' in the type declaration has to refer to the *same*
type. 'a' gets bound to a specific type and must match that type
throughout. 

now, f '0' = 0 has type f::Num t => Char -> t. This means that f takes
a char and returns some numeric type. But char is not an instance of
Num, and so this definition of f does not match its type signature.

> 3
> 
> Why is the following legal
> 
>   [] :: Num a => [a]
> 
> but not with a self declared class instad of Num (unresolved overloading)?

there was a discussion here over the last few days about the
polymorphism of []. It might help to read over that.

> 
> 4
> 
> Why does in the declaration
> 
>   f undefined = '0'
> 
> the expression undefined work apparently in the same way as a wildcard?

I believe that what you are doing here is binding the argument of f to
a local name "undefined". You are subsequently not using the argument
to f, so it is ignored giving a wildcard type of behavior. If this was
included in a list of pattern-matching definitions of f, then it will
match when all others fail. But this is not a property of "undefined"
in this context. You've just given it a name that happens to coincide
with a reserved word. You could just as easily use "x" or "foo" or
whatever in the same way. Note that commonly, if you are not going to
use the argument to a function, you use "_" to prevent it from being
bound at all. 

for example, in:

f:: Int -> Char
f 1 = '1'
f 2 = '2'
f undefined = '0' -- using your word here, but I'd typically not since
its a reserved word

you could replace "undefined" with "_"

f _ = '0'

to get the same result. The first usage will generate a compiler
warning for the unused binding. The second will not. 

At least that's how I understand it.

A
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