On Thu, Nov 11, 2010 at 9:24 AM, 贾旭卿 wrote:

This is exercise 3.1 of Real World Haskell. I have my length function like this:

myLength [] = 0 myLength (_:xs) = 1 + (myLength xs)

And I assumed the type signature is like this: mylength :: [a] -> Num

But when I wrote this into the file and reloaded it into ghci, there is an error.

...

    The type signature for `mylength' lacks an accompanying binding Failed, modules loaded: none.

And the type signature given by ghci is

...

myLength :: (Num t1) => [t] -> t1

So how can I modify the function to have a type signature like the first one?

You can't, since Num isn't a type, it's a typeclass.

myLength :: (Num b) => [a] -> b

means that myLength takes a list of any type and can return any type that is an instance of Num (Num being the typeclass of numbers, that means that you can do most things you do on numbers, adding them, multiplying them, and so on...). If you want a simpler type signature, you could use :

myLength :: [a] -> Int

or

myLength :: [a] -> Integer

since Int (32 or 64 bits integer) and Integer are real type that are instances of the Num typeclass. -- Jedaï

On Thursday 11 November 2010 09:24:44, 贾旭卿 wrote:

This is exercise 3.1 of Real World Haskell. I have my length function like this:

myLength [] = 0 myLength (_:xs) = 1 + (myLength xs)

And I assumed the type signature is like this: mylength :: [a] -> Num

But when I wrote this into the file and reloaded it into ghci, there is an error.

...

The type signature for `mylength' lacks an accompanying binding Failed, modules loaded: none.

That's because you mistyped the function name in the type signature, the funcion has an uppercase L, the signature got a lowercase l. Without that typo, you'd have gotten an error: Class `Num' used as a type In the type signature for `myLength': myLength :: [a] -> Num which would probably have helped.

And the type signature given by ghci is

...

myLength :: (Num t1) => [t] -> t1

So how can I modify the function to have a type signature like the first one?

Jedaï answered that.