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Today's Topics:
1. Re: How to show a predicate (Kostiantyn Rybnikov)
2. Re: explaining effects (Alexander Berntsen)
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Message: 1
Date: Tue, 29 Dec 2015 09:21:52 +0200
From: Kostiantyn Rybnikov <k...@k-bx.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] How to show a predicate
Message-ID:
<CAAbahfQZbsP+b=zaom6obkhcsf85ejh6iabhvjpteugyute...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
You can store both values, a function and a string representation, in a
predicate:
data Predicate = Predicate (a -> Bool) String
And write corresponding Show instance to show string.
On Wed, Dec 23, 2015 at 2:55 PM, martin <martin.drautzb...@web.de> wrote:
> Hello all,
>
> in my program, I do stuff with predicates (a->Bool). For the most part
> this representation is just fine, but at the very
> end I need to convert a resulting predicate into a String so I can write
> it to a file.
>
> Wenn I represent my predicates as Lists or Sets, then this is doable and I
> am tempted to do it this way. The only other
> option I could come up with was to have a representation of "everything",
> which would in my case be large (10^8) but
> finite. Then I could construct a List or a Set at the very end, as [x |
> x<-everything, p x] without having explicit sets
> in the intermediate steps.
>
> I cannot see any other option, but I thought I better ask.
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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Message: 2
Date: Tue, 29 Dec 2015 12:00:50 +0100
From: Alexander Berntsen <alexan...@plaimi.net>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] explaining effects
Message-ID: <568267e2.8050...@plaimi.net>
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Rein explains effects well enough. Now for side-effects: a side-effect
is what is allowed to happen in the absence of a weak equivalence
between call-by-name (including memoised versions) and call-by-value[0].
Let's play "spot the side-effect!"
- -- Haskell function for adding two numbers.
f x y = x + y
- -- function in a language that happens to be syntactically like Haskell.
f x y = print "Hallo!" >> x + y
- -- program to evaluate the function.
- -- it just happens to be equal in Haskell and the Haskell-like language.
main = let a = f 4 2 in print a >> print a
evaluation-strategy | call-by-need | call-by-value
- -------------------------------------------------------------
x + y | 6\n6 | 6\n6
print "Hallo" >> x + y | Hallo!\n6\n6 | Hallo!\n6\nHallo!\n6
See how the latter function differs in call-by-need and call-by-value?
This is because it has a *side-effect* of printing stuff. This is not
allowed in Haskell. And in fact, the latter function is impossible in
Haskell.
As Rein was getting at by abstraction leaks -- side-effects have an
observable interaction with the outside world.
[0] <https://www.cs.indiana.edu/~sabry/papers/purelyFunctional.ps>
- --
Alexander
alexan...@plaimi.net
https://secure.plaimi.net/~alexander
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