[Haskell-cafe] Non-deterministic function/expression types in Haskell?
Brandon Allbery
allbery.b at gmail.com
Fri Jan 12 18:00:17 UTC 2018
You seem rather confused as to what a monad is. It is not about "everything
is in an IORef", for one. (IO is not the only Monad, nor are the others
pretending to be IO.)
On Fri, Jan 12, 2018 at 11:38 AM, Benjamin Redelings <
benjamin.redelings at gmail.com> wrote:
> Hi Oleg,
>
>
> On 01/11/2018 09:17 AM, Oleg Grenrus wrote:
>
>> Hi Benjamin
>>
>> Let's see what you ask for, you have *new* syntax for types:
>>
>> a[N] and a[D]
>>
>> what are a[N][N] or a[N][D] or a[N][D] or a[D][D]?
>>
>> Aren't they a[N], a[N], a[N] and a[D] respectively?
>> That's what monads are about!
>>
> Just to be clear, I'm not using [N] as an operator on types, but as part
> of the type. So a type could be something like the pair (Int,D) or
> (Int,N). In that context a[N][N] is not part of the system.
>
>>
>> So
>>
>> a[N] ~ Distr a
>> a[D] ~ Identity a ~ a
>>
>> No need to complicate type-system! You just to not be afraid of monads!
>>
>> Monads aren't sequencing, they are computational context.
>>
>> I guess, you just want more natural term-level syntax.
>>
>> You can use ApplicativeDo [1] (in GHC-8.0+), so e.g.
>>
>> do x <- normal 0 1
>> y <- normal 0 1
>> return (f x y)
>>
>> will be transformed into
>>
>> liftA2 f (normal 0 1) (normal 0 1)
>>
>> That's almost like
>>
>> f (normal 0 1) (normal 0 1)
>>
>> if you have proper syntax highlighting ;)
>>
>> Note: various term syntax extensions been proposed.
>> E.g. idiom brackets in the "Applicative programming with effects" [2]:
>>
>> (| f (normal 0 1) (normal 0 1) |)
>>
>> to mean
>>
>> pure f <*> normal 0 1 <*> normal 0 1
>>
>> which is equivalent to above liftA2 expression. If you like that, you
>> can check
>> "the Strathclyde Haskell Enhancement", it supports idiom brackets.
>>
> In my other message I posted an example that doesn't fit this very well:
>
> do { x <- f x } does not work, where as let x = f x does work. Basically
> I'm trying to avoid monads because I want to use the full features of the
> Haskell language, instead of programming in an embedded language. In that
> context "more natural" term-level syntax is not sufficient.
>
> Also, it seems possible that everything in Haskell COULD be written in a
> monad. We could eliminate recursive let bindings, and tell people to
> create a giant state machine which they use by reading and writing IORefs.
> But then you also eliminate some of the point of using Haskell and may as
> well go write in C or something. So it seems to me that just because you
> CAN use a monad doesn't mean you SHOULD use a monad, and the question is
> "when is a monad better than something else?"
>
> Does that make sense? Am I missing something?
>
>
> -BenRI
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--
brandon s allbery kf8nh sine nomine associates
allbery.b at gmail.com ballbery at sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net
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