[moved to Haskell-cafe] This makes my head hurt! Let me see if I get this right. When the type checker sees: build' as a a2 ... then it invokes the second instance of BuildList (i.e. BuildList a (a->r)) and returns the value: build'(a:as) a2 ... But when it sees just: build' as a (as a value, not used as a function application), then the type checker invokes the first instance and returns: reverse$ a:as So they key here appears to be that the type-checker can distinguish between function application and other uses of an expression? #g -- At 18:18 01/06/04 -0700, oleg@pobox.com wrote:
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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