[Haskell-cafe] Bool is not...safe?!
aquagnu at gmail.com
Thu Jul 5 15:04:50 UTC 2018
So, the problem is that test of emptiness does not force you to
something right. And possible errors are:
# do if NOT empty - BUG!
# do if EMPTY - BUG TOO!
# do if NOT empty - BUG!
if NOT empty:
# now nothing or old "do if NOT EMPTY"
OK, I understand it. But how is it related to Booleans? :) Sure, if you
use Maybe or Either you are forced with nature of ">>=": it cuts off
incorrect branches. With if-then - it does not. But it's not related to
Bool: Bool is result for predicates. Maybe/Either forces you with magic
operation ">>=" (which is hidden by do-sugar). Bool does not force you -
right. But it's problem of Haskell implementation. For example, Prolog
Haskell forces you in monad "do":
someInt <- someMaybe
-- will not be executed if someMaybe is Nothing
Prolog forces you too but on success/fail (Boolean?):
someGoal, anotherGoal % anotherGoal will not be executed if
someGoal is False
Haskell adds only "bool" function which is close to ">>=" in terms of it
hides semantic of right bool's processing, avoid those possible errors.
If you will use "bool" anywhere when you use Bool as result - all will
be fine, or? Sure, you can move "head" usage out of "bool" but you will
get empty "bool"s argument. So, IMHO examples of problem with booleans
is not related to Bool type at whole, but related to problem that Bool
has kind * but not * -> * to be processed in monadic style (and to has
needed ">>=" semantic to force you).
OK, but original article was not about Haskell's monads, but about Bool
in general :) Also what I can't understand: if we will think in such
manner does it mean that "if..test" construct is "boring"/"blindness"
(excuse my English:) at whole? And all predicates must be removed from
the language? What will happen to `filter` function without predicates?
And no way to avoid "if..else" construct and predicates functions.
As for me, this question is related to static-types fanaticism and "How
many angels could dance on the head of a pin". Example with "head" is
totally incorrect - it can deconstruct list and no need to predicate
function. But what I should do with isSpace, isLower, etc? How to use
predicates at whole? :) To map a -> Bool to a -> Maybe a ?
What about function which returns IO Bool? Action which can ends with
non-critical failure (and need optionally logging, for example) ?
05.07.2018 17:27, Stefan Monnier wrote:
>> There is an opinion that Bool type has problems. It's "dangerous", because
>> it's not good to be used as flag for success/fail result. I read this post:
>> https://existentialtype.wordpress.com/2011/03/15/boolean-blindness/ and was
>> shocked. How popular is such opinion? Is it true, that bool is "bad" type?
> To me the argument boils down to the `head` case mentioned by Alex.
> Most programming languages force you to write code like
> if List.empty l then
> ... x = List.head l ...
> where the problem is that the fact that the List.head call will find the
> list non-empty is not obvious (in the sense that it requires reasoning).
> In contrast
> case l
> | nil => ...
> | cons x xs => ...
> makes it trivially obvious that `x` is extracted from a non-empty list
> without any reasoning needed at all.
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