[clean-list] Parametrizing instance declarations

[John van Groningen]

David Noris wrote:
>How might one define the following three class instances without duplicating code?  My attempts to parametrize the declaration fail with errors such as
>
>(1) type variable of type of lifted argument b appear in the specified type
>(2) internal overloading of "_createArray" could not be solved.
>
>----------------------------------------------
>
>class copyElt e :: {#e} Int -> e
>
>instance copyElt Char
>where
>   copyElt a i = c.[0]
>   where
>       c :: {#Char}
>       c = {a.[i]}
>
>instance copyElt Int
>where
>   copyElt a i = c.[0]
>   where
>       c :: {#Int}
>       c = {a.[i]}
>
>instance copyElt Real
>where
>   copyElt a i = c.[0]
>   where
>       c :: {#Real}
>       c = {a.[i]}

You could use a macro (or function) and use it in each instance declaration:

copyEltCharIntOrReal a i :== (c a).[0]
    where
        c :: {#e} -> {#e} | Array {#} e
        c a = {a.[i]}

instance copyElt Char where
    copyElt a i = copyEltCharIntOrReal a i

instance copyElt Int where
    copyElt a i = copyEltCharIntOrReal a i

instance copyElt Real where
    copyElt a i = copyEltCharIntOrReal a i

Unfortunately this doesn't work:

copyEltCharIntOrReal a i :== c.[0]
    where
        c = {a.[i]}

because the compiler cannot determine what kind of array ({},{!} or {#})
has to be created. Therefore the type of c is specified. The argument a
is added to c, because the type c depends on a.

In future releases of Clean it will be possible to use:

copyEltCharIntOrReal a i :== c.[0]
    where
        c = {#a.[i]}

to indicate that an unboxed array has to be created.

Kind regards,

John van Groningen