[Haskell-cafe] Uses of `fix' combinator
Miguel Mitrofanov
miguelimo38 at yandex.ru
Thu Feb 19 09:31:08 EST 2009
Each data type in Haskell contains one element, which is usually
"invisible". It's called "bottom" and denoted by (_|_).
Naturally (_|_) of type Int and (_|_) of type Char are different;
however, they are denoted as if they are the same, 'cause there isn't
much difference between them. Anyway, if you try to calculate
something which happens to be (_|_) and your program would throw an
error or loop forever.
Now, since there is (_|_)::Int and (_|_)::Char, there are also ((_|_),
(_|_)) :: (Int, Char) as well as (1, (_|_)) :: (Int, Char) and ((_|_),
'a') :: (Int, Char); all of them are different from (_|_) :: (Int,
Char). If a value contains (_|_) somewhere inside it, we say that it
is less defined than the value obtained from it by replacing (_|_)s
with something else. For example, (_|_) is less defined than ((_|_),(_|
_)), which is less defined than (1, (_|_)) or ((_|_), 'a'); and both
of them are less defined than (1, 'a').
'fix' is a function which maps a function 'f' to the LEAST defined x
such that f x = x. Such 'x' always exists; it could be (_|_), but it
could be something else. For example, (^^2) is a strict function,
which means that (_|_)^^2 = (_|_); therefore fix (^^2) = (_|_) - which
you've discovered yourself.
A stupid example: fix (\a -> (1, snd a)) = (1, (_|_)). (_|_) is not
the right answer: (\a -> (1, snd a)) (_|_) = (1, snd (_|_)) = (1, (_|
_)) which isn't (_|_).
Another, less stupid example: fix (\a -> (1, fst a)) = (1, 1) - which
doesn't contain (_|_) anywhere inside it. See, (_|_) is not the right
answer here: (\a -> (1, fst a)) (_|_) = (1, fst (_|_)) = (1, (_|_)),
which isn't (_|_). But (1, (_|_)) is not the right answer either: (\a -
> (1, fst a)) (1, (_|_)) = (1, fst (1, (_|_))) = (1, 1).
On 19 Feb 2009, at 17:00, Khudyakov Alexey wrote:
> Hello,
>
> While browsing documentation I've found following function
>
>> -- | @'fix' f@ is the least fixed point of the function @f@,
>> -- i.e. the least defined @x@ such that @f x = x at .
>> fix :: (a -> a) -> a
>> fix f = let x = f x in x
>
> I have two questions. How could this function be used? I'm unable to
> imagine
> any. Naive approach lead to nothing (no surprise):
>
> Prelude Data.Function> fix (^^2)
> <interactive>: out of memory (requested 2097152 bytes)
>
>
> Second question what does word `least' mean?`a' isn't an Ord instance.
>
> --
> Khudyakov Alexey
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