[Haskell-cafe] data type declaration

[Vo Minh Thu]

2010/7/25 Andrew Coppin <andrewcoppin at btinternet.com>:
> Patrick Browne wrote:
>>
>> Andrew,
>> Thanks for your detailed feedback, it is a great help.
>>
>
> Well, I like to be helpful.
>
>> I appreciate that the code does not do anything useful, nor is it an
>> appropriate way to write Haskell, but it does help me
>> understand language constructs.
>
> Personally, I find it easier to understand things when they do something
> meaningful, but sure.
>
>> I am studying the Haskell type class system as part of a language
>> comparison. I am trying to exercise and understand the constructs rather
>> than develop a meaningful application.
>>
>
> The best way to understand Haskell is... to completely forget everything you
> already know, and start again from scratch. ;-) Still, I gather that's not
> the point of this particular exercise.
>
> Since you're interested in comparisons... A method is simply a way of giving
> the same name to several different functions, and have the compiler pick the
> correct one based on the argument types.
> [snip]

Actually in Haskell, the choice of the correct definition is not only
based on the argument types. It could be based on the result type:

class C a where
  f :: a
  g :: a -> Int
  h :: String -> a

An example of the 'h' case is simply the 'read' function:

Prelude> :t read
read :: (Read a) => String -> a

For instance:

Prelude> read "1" :: Int
1
Prelude> read "1" :: Double
1.0

Note that usually, the type annotation is not needed because the
compiler has enough information from the context to infer it.

Cheers,
Thu