[Hugs-users] the built-in function enumFrom

[Daniel Fischer]

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
>
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> Paqui Lucio
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