[Haskell] Re: how to write a list builder? fixpoint?

[Ben.Yu at combined.com]

Thanks Oleg.

This is really a nice example. Never even dream about such wonderful use of
type classes!

It seems to me that the "fallow-undecidable-instances" for this example is
not necessary though.

It compiles with just glasgow-exts

Another question about overloading with type classes. It seems that these
overloaded functions cannot be passed as higher-order function. Is that
true?A higher order function can never be overloaded?

In an example, how can I pass "build" as a function to another function
that does some algorithm? (as the traditional Builder pattern in OO term)

myalg builder = ...... build a b c ......

Ben.



                                                                                                                                       
                      oleg at pobox.com                                                                                                   
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                      06/01/2004 08:18                                                                                                 
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> Is it possible to write a function to build a list [a]?
> so that I can write [a,b,c,d] as "getBuilt $ build a b c d"?

Yes, in the format very close to desired.

> {-# OPTIONS -fglasgow-exts #-}
> {-# OPTIONS -fallow-undecidable-instances #-}
>
> module Foo where
>
> class BuildList a r  | r-> a where
>     build' :: [a] -> a -> r
>
> instance BuildList a [a] where
>     build' l x = reverse$ x:l
>
> instance BuildList a r => BuildList a (a->r) where
>     build' l x y = build'(x:l) y

That's it. It works both on GHC and Hugs.

*Foo> build' [] True :: [Bool]
[True]
*Foo> build' [] True False :: [Bool]
[True,False]
*Foo> build' [] True False False :: [Bool]
[True,False,False]
*Foo> build' [] 'a' 'b' 'c' 'd' 'e' :: [Char]
"abcde"
*Foo> build' [] (1::Int) :: [Int]
[1]
*Foo> build' [] (1::Int) (2::Int) :: [Int]
[1,2]
*Foo> build' [] (1::Int) (2::Int) (3::Int) :: [Int]
[1,2,3]

Note that the type annotation [Bool] etc. at the end is required: it
is the delimiter of the list. Who would have thought that the type
annotation can play the role of Nil...

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