Am Donnerstag 29 Oktober 2009 11:06:31 schrieb Paqui Lucio:
Hi, In Prelude.hs you can find the definition enumFrom x = map toEnum [ fromEnum x ..]
That's the default implementation, when an instance declaration doesn't provide an explicit method.
Load a file.hs of just one line: data TBool = Falsity | Undefined |Truth deriving (Enum, Show) Then, I evaluate enumFrom Falsity and, as expected, it gives [Falsity,Undefined,Truth] However, the expression map toEnum [ fromEnum Falsity ..] gives first an ambiguity problem
Of course. [fromEnum Falsity .. ] is a list of Ints, namely [0 .. ] (which is [0 .. maxBound :: Int]). Mapping toEnum over that list can give a list of any type belonging to Enum, so it must somehow be specified which type to use (Bool, Int, Integer, (), Char, ...)
and then the problem I expected Main> map toEnum [ fromEnum Falsity ..] ERROR - Unresolved overloading *** Type : Enum a => [a] *** Expression : map toEnum (enumFrom (fromEnum Falsity)) Main> map toEnum [ fromEnum Falsity ..]::[TBool] [Falsity,Undefined,Truth, Program error: {_toEnum TBool_Falsity 3} (98 reductions, 301 cells) Why the function enumFrom works perfectly in spite of its erroneous definition?
Because the implementation is smarter than the default method :) Given a type with k constructors, the implementation will, upon a clause "deriving Enum", create methods enumFrom(Then) that work, probably by doing something like enumFrom val = map toEnum [fromEnum val .. k-1]
Thanks in advance, Paqui P.D. I am using the
|| || || || || || ||__ Hugs 98: Based on the Haskell 98 standard || ||___|| ||__|| ||__|| __|| Copyright (c) 1994-2002 ||---|| ___|| World Wide Web: http://haskell.org/hugs || || || Report bugs to: hugs-bugs@haskell.org || || Version: Nov 2002
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