Hello John,
just interpreting what you mean (but I may misunderstand it completely).
With a list comprehension you can formulate:
(just a simple example)
Prelude> take 6 $ [ (^2) x | x <- [0..], 0 == x `mod` 2]
[0,4,16,36,64,100]
In J:
(*:#~0=2&|) i.12
0 4 16 36 64 100
Perhaps something like this?
Jon Hough schreef op 27-12-14 om 15:26:
Hi Aai,
I see what you are saying, but I would say that implicitly inside f there is some sort of
"such that" syntax.
My motivation for asking for a "such that / where" construct is to create
computable math definitions, as in Haskell.
for example, if I want the set (or group):
{ x e. Sym(4) | order(x) = 4} (where Sym(4) is the symmetric group on 4 letters)
then, assuming I have a verb "sym" and a verb "order" I can do
(sym 4) where (4&=@:order)
Then it may just be possible to generate groups from group presentations (
http://en.wikipedia.org/wiki/Presentation_of_a_group)(I haven't figured out how
to do this, I am just thinking about it over the holidays)
Date: Sat, 27 Dec 2014 08:46:12 +0100
From: agroeneveld...@gmail.com
To: programm...@jsoftware.com
Subject: Re: [Jprogramming] "Such that" syntax in J
Sorry for jumping in late.
The Haskell expression you showed here is called a list comprehansion
which can be easily formulated as
map f xs
and this is in J simply:
f xs
Jon Hough schreef op 26-12-14 om 17:04:
In Haskell (and other languages I'm sure) one can express the idea of "such
that" to denote one expression depending on another. In Haskell we can use the
syntax:
[ f x | x <- xs ]
which means
"the list of all f x
such that x is drawn from xs."
So | is syntactically similar to the English "such that". (and very much like |
in mathematical sets)
I would like to know if J has a construct to express the above Haskell code.
Thanks,
Jon.
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