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Today's Topics:
1. Question about precedence (Marc Gorenstein)
2. Re: Question about precedence (Brandon Allbery)
3. Re: Question about precedence (Darren Grant)
4. Re: Question about precedence (Michael Orlitzky)
5. Still confused (Marc Gorenstein)
6. Re: Still confused (Ryan Bell)
7. Re: Still confused (Brandon Allbery)
8. Re: Still confused (Michael Peternell)
----------------------------------------------------------------------
Message: 1
Date: Mon, 1 Jul 2013 12:14:04 -0400
From: Marc Gorenstein <[email protected]>
Subject: [Haskell-beginners] Question about precedence
To: [email protected]
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<camqvbq5bc8vbd743qnyrpbz0+khmythakvotpvxt3cffi5a...@mail.gmail.com>
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What sort of beast is ( / 8) in Haskell? It looks like it is a function
that divides a number by 8.
*Main> ( / 8 ) 4
0.5
*Main> let a = ( / 8 )
*Main> a 4
0.5
-- Yup that works.
Does ( / 8 ) turn into a function that takes an argument, so that the
"left" input to the /, becomes a right input to the function?
What precedence rule is being followed here?
Thanks,
Marc
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Message: 2
Date: Mon, 1 Jul 2013 12:32:07 -0400
From: Brandon Allbery <[email protected]>
Subject: Re: [Haskell-beginners] Question about precedence
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
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On Mon, Jul 1, 2013 at 12:14 PM, Marc Gorenstein
<[email protected]>wrote:
> What sort of beast is ( / 8) in Haskell? It looks like it is a function
> that divides a number by 8.
>
It's an operator section.
http://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-300003.5describes
their syntax and their precedence behavior.
--
brandon s allbery kf8nh sine nomine associates
[email protected] [email protected]
unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net
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Message: 3
Date: Mon, 1 Jul 2013 09:34:01 -0700
From: Darren Grant <[email protected]>
Subject: Re: [Haskell-beginners] Question about precedence
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
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I think that since infix notation always takes two parameters, the compiler
handles parameter swapping to match the unfilled argument for convenience.
Not sure where this would be described in a spec though.
Cheers,
Darren
On Mon, Jul 1, 2013 at 9:14 AM, Marc Gorenstein
<[email protected]>wrote:
> What sort of beast is ( / 8) in Haskell? It looks like it is a function
> that divides a number by 8.
>
> *Main> ( / 8 ) 4
> 0.5
>
> *Main> let a = ( / 8 )
> *Main> a 4
> 0.5
>
> -- Yup that works.
>
> Does ( / 8 ) turn into a function that takes an argument, so that the
> "left" input to the /, becomes a right input to the function?
> What precedence rule is being followed here?
>
> Thanks,
>
> Marc
>
>
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
>
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Message: 4
Date: Mon, 01 Jul 2013 12:44:15 -0400
From: Michael Orlitzky <[email protected]>
Subject: Re: [Haskell-beginners] Question about precedence
To: [email protected]
Message-ID: <[email protected]>
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On 07/01/2013 12:14 PM, Marc Gorenstein wrote:
> What sort of beast is ( / 8) in Haskell? It looks like it is a function
> that divides a number by 8.
>
Yep. This is probably easier to think about with addition rather than
division.
So, (+) is a function. It takes two arguments, and adds them. You need
the parenthesis to prevent it from being an infix operator (used in
*between* the numbers), but that's just a detail. Try it in ghci:
ghci> (+) 1 2
3
Thanks to currying, you can construct a function called "plus_one" by
leaving off the last argument:
ghci> let plus_one = (+) 1
ghci> let plus_one = (+ 1)
Either notation will work, but they do subtly different things: in the
first case, you get 1 + x, and in the second, you get x + 1. Nevertheless.
ghci> plus_one 2
3
Division works the same way.
------------------------------
Message: 5
Date: Mon, 1 Jul 2013 18:03:06 +0000 (UTC)
From: Marc Gorenstein <[email protected]>
Subject: [Haskell-beginners] Still confused
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
Hi Brandon, Darren, and Michael,
Thanks for you responses, but I'm still confused.
Here are two examples of operator sections. The first takes the infix operator
/ and turns it into a prefix operator.
Prelude> let eight_div_by = ((/) 8 )
Prelude> eight_div_by 4
2.0
I get that. But look at the following: We now have a prefix operator with
the input on the "wrong" side.
Prelude> let div_by_eight = ( / 8 )
Prelude> div_by_eight 4
0.5
Why should ( / 8) 4 = 0.5?
Thanks again,
Marc
------------------------------
Message: 6
Date: Mon, 1 Jul 2013 14:19:10 -0400
From: Ryan Bell <[email protected]>
Subject: Re: [Haskell-beginners] Still confused
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Message-ID:
<cacad8sr6ex1kwun1bga-odexdcqtiwzbfahajs21ew2udea...@mail.gmail.com>
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Does it help to think of it like algebra?
let x = 4
where x / 8
On Mon, Jul 1, 2013 at 2:03 PM, Marc Gorenstein
<[email protected]>wrote:
> Hi Brandon, Darren, and Michael,
>
> Thanks for you responses, but I'm still confused.
>
> Here are two examples of operator sections. The first takes the infix
> operator
> / and turns it into a prefix operator.
>
> Prelude> let eight_div_by = ((/) 8 )
> Prelude> eight_div_by 4
> 2.0
>
> I get that. But look at the following: We now have a prefix operator with
> the input on the "wrong" side.
>
> Prelude> let div_by_eight = ( / 8 )
> Prelude> div_by_eight 4
> 0.5
>
> Why should ( / 8) 4 = 0.5?
>
>
> Thanks again,
>
> Marc
>
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
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Message: 7
Date: Mon, 1 Jul 2013 14:20:19 -0400
From: Brandon Allbery <[email protected]>
Subject: Re: [Haskell-beginners] Still confused
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Message-ID:
<cakfcl4u_rgs4-tbmjwxf5yfqgqair7kbdzothaje_9nj1bs...@mail.gmail.com>
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On Mon, Jul 1, 2013 at 2:03 PM, Marc Gorenstein
<[email protected]>wrote:
> Here are two examples of operator sections. The first takes the infix
> operator
> / and turns it into a prefix operator.
>
> Prelude> let eight_div_by = ((/) 8 )
> Prelude> eight_div_by 4
> 2.0
>
Yes. A section is just this conversion of an infix operator to a prefix
function, but with one parameter carried along with it. You can supply it
on either side as appropriate; (/ 8) is the same as (\x -> x / 8) which in
turn is the same as (\x -> (/) x 8), whereas (8 /) is (\x -> 8 / x) is (\x
-> (/) 8 x). Note that it must be *inside* the parentheses to be a section;
if it's outside, then it's a normal function (not operator!) parameter.
--
brandon s allbery kf8nh sine nomine associates
[email protected] [email protected]
unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net
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Message: 8
Date: Mon, 1 Jul 2013 20:23:43 +0200
From: Michael Peternell <[email protected]>
Subject: Re: [Haskell-beginners] Still confused
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
(/ 8)
is the same as
(\x -> x / 8)
you can think of it as
(HERE_IS_SOMETHING_MISSING / 8)
whereas
(/) 8
is normal function currying, i.e. it is the same as
(\x -> (/) 8 x)
or
(\x -> 8 / x)
just like
foo y
is the same as
(\x -> foo y x)
if foo is a function that takes 2 parameters.
on the other hand
(8 /)
is like
(8 / HERE_IS_SOMETHING_MISSING)
which translates to
(\x -> 8 / x)
Am 01.07.2013 um 20:03 schrieb Marc Gorenstein <[email protected]>:
> Hi Brandon, Darren, and Michael,
>
> Thanks for you responses, but I'm still confused.
>
> Here are two examples of operator sections. The first takes the infix
> operator
> / and turns it into a prefix operator.
>
> Prelude> let eight_div_by = ((/) 8 )
> Prelude> eight_div_by 4
> 2.0
>
> I get that. But look at the following: We now have a prefix operator with
> the input on the "wrong" side.
>
> Prelude> let div_by_eight = ( / 8 )
> Prelude> div_by_eight 4
> 0.5
>
> Why should ( / 8) 4 = 0.5?
>
>
> Thanks again,
>
> Marc
>
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
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