[Haskell-cafe] Syntax proposal for "reverse apply"/"pipeline apply" (flip ($))

[Clark Gaebel]

Last I checked,

(&) = flip ($)

is both shorter to type, and more explicit than:

import Control.Apply.Reverse

  - Clark


On Thu, Apr 17, 2014 at 2:03 PM, Hans Höglund <hans at hanshoglund.se> wrote:

>
> There is also is my humble attempt to standardize the (&) formulation:
>
> http://hackage.haskell.org/package/reverse-apply
>
> As you can see, these definitions now mirror those in 'lens' exactly. I
> see no reason why this definition should not move to base. IMHO, the
> Diagrams definition is a very specific one based on the needs of that EDSL,
> while the lens formulation is the expected one.
>
> Mvh,
> Hans
>
>
> On 17 apr 2014, at 14:00, haskell-cafe-request at haskell.org wrote:
>
> Message: 1
> Date: Wed, 16 Apr 2014 16:49:31 -0700
> From: Dan Burton <danburton.email at gmail.com>
> To: Alexey Muranov <alexey.muranov at gmail.com>
> Cc: haskell-cafe <haskell-cafe at haskell.org>
> Subject: Re: [Haskell-cafe] Syntax proposal for "reverse
> apply"/"pipeline apply" (flip ($))
> Message-ID:
> <CALSygwf0wdBbrfQeED_raWjNfttYaX7pwDTzmBTTHaUqEkX5rg at mail.gmail.com>
> Content-Type: text/plain; charset="utf-8"
>
>
> In the "lens" package, this is (&) at infixl 1. In the "diagrams" package,
> this is (#) at infixl 8. You're certainly not the first to want this, but
> nobody can ever agree what it should be called or what fixity it should
> have. You can always just define it yourself.
>
> -- Dan Burton
>
>
> On Wed, Apr 16, 2014 at 3:25 PM, Alexey Muranov <alexey.muranov at gmail.com
> >wrote:
>
> Hello,
>
>
> i am completely new to Haskell, but i am somewhat fascinated by
>
> lambda-calculus and programming.
>
>
> For whatever it is worth, i would like to propose for discussion a syntax
>
> for "(flip ($))" operation in Haskell.
>
>
> I think that a good syntax would be "|^", for example:
>
>
>    square x = x * x
>
>    y = 3 |^ square             -- y == 9
>
>
>
> Explanation:
>
>
> * i would have suggested just ^, but it would conflict with number
>
> exponentiation,
>
>
> * it is rather common in mathematics to write function application in
>
> exponential notation:  x ^ f  instead of  f(x), especially if  f  is an
>
> automorphism of some structure,
>
>
> * (flip ($)) is exactly the exponentiation of Church numerals,
>
>
> * in "The calculi of lambda-conversion", Alonzo Church uses the
>
> "shorthand" notation "[N^M]" for "(MN)", where M and N are lambda-terms.
>
>
> * I am probably not the only person missing the ability to apply functions
>
> from the right:
>
>
>
> http://stackoverflow.com/questions/1457140/haskell-composition-vs-fs-pipe-forward-operator
>
>
> Well, other notations i've thought of are "\^" and "~$".
>
>
> Alexey.
>
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>
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>
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>
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> ------------------------------
>
> Message: 2
> Date: Wed, 16 Apr 2014 16:56:37 -0700 (PDT)
> From: Alexey Muranov <alexey.muranov at gmail.com>
> To: haskell-cafe at googlegroups.com
> Cc: haskell-cafe <haskell-cafe at haskell.org>
> Subject: Re: [Haskell-cafe] Syntax proposal for "reverse
> apply"/"pipeline apply" (flip ($))
> Message-ID: <0569e94e-44b8-44a0-abe3-4bd7d80b2ed4 at googlegroups.com>
> Content-Type: text/plain; charset="utf-8"
>
>
> On Thursday, April 17, 2014 1:49:31 AM UTC+2, Dan Burton wrote:
>
> In the "lens" package, this is (&) at infixl 1. In the "diagrams" package,
>
>
> this is (#) at infixl 8. You're certainly not the first to want this, but
>
> nobody can ever agree what it should be called or what fixity it should
>
> have. You can always just define it yourself.
>
>
>
> (#) does not look too bad either.
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> ------------------------------
>
> Message: 3
> Date: Wed, 16 Apr 2014 18:41:01 -0700
> From: Dan Burton <danburton.email at gmail.com>
> To: Alexey Muranov <alexey.muranov at gmail.com>
> Cc: haskell-cafe <haskell-cafe at haskell.org>,
> haskell-cafe at googlegroups.com
> Subject: Re: [Haskell-cafe] Syntax proposal for "reverse
> apply"/"pipeline apply" (flip ($))
> Message-ID:
> <CALSygwdyxhNLafMZDm15iVzPO4qYytdC7q2fDRhabNWJ4Vrb=g at mail.gmail.com>
> Content-Type: text/plain; charset="utf-8"
>
>
> Interesting. I've never seen it proposed as right-associative before. Just
> FYI, the implementation is as simple as this:
>
> infixr 1 |^
> (|^) :: a -> (a -> b) -> b
> x |^ f = f x
>
> Then you can write:
>
> 3 |^ 2 |^ (^) -- produces 2^3 = 8
>
> It seems very odd to me. I don't know why you'd want to apply the arguments
> backwards one by one. However, lens and diagrams both provide examples
> where you want to start with a value and apply functions "forwards" one by
> one, which is why their corresponding operators are left-associative.
>
> -- Dan Burton
>
>
>
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>


-- 
Clark.

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