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Re: how to compute one and filter it immediately rather than calculated all and then filter the whole

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meInvent bbird <jobmattcon@gmail.com> writes:

>  [g x | x <- [1..20], f x] 
>
> i do not understand why i do not specify to filter true in f x
> and it can get the true?
>
> how do it know i must choose true?

Just because that's what the syntax means.  After the | you write
generators (like x <- [1..2]) or Boolean guards that refine what
generated values are permitted.  For example

  [i | i <- [1..20], True]

is just [1..20] and

  [i | i <- [1..20], False]

is just [].

Note that writing the filtered list like this may be slower than the
original version you had.  It all depends on how the calculation of the
pairs is done.  Your example is rather complicated and that's putting me
off looking at the details.

<snip>
-- 
Ben.

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how to compute one and filter it immediately rather than calculated all and then filter the whole meInvent bbird <jobmattcon@gmail.com> - 2016-07-21 05:07 -0700
  Re: how to compute one and filter it immediately rather than calculated all and then filter the whole Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-07-21 16:31 +0100
    Re: how to compute one and filter it immediately rather than calculated all and then filter the whole meInvent bbird <jobmattcon@gmail.com> - 2016-07-21 19:29 -0700
      Re: how to compute one and filter it immediately rather than calculated all and then filter the whole meInvent bbird <jobmattcon@gmail.com> - 2016-07-21 19:47 -0700
        Re: how to compute one and filter it immediately rather than calculated all and then filter the whole meInvent bbird <jobmattcon@gmail.com> - 2016-07-21 21:38 -0700
          Re: how to compute one and filter it immediately rather than calculated all and then filter the whole Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-07-22 09:40 +0100
            Re: how to compute one and filter it immediately rather than calculated all and then filter the whole meInvent bbird <jobmattcon@gmail.com> - 2016-07-22 01:57 -0700
              Re: how to compute one and filter it immediately rather than calculated all and then filter the whole meInvent bbird <jobmattcon@gmail.com> - 2016-07-22 02:02 -0700
                Re: how to compute one and filter it immediately rather than calculated all and then filter the whole meInvent bbird <jobmattcon@gmail.com> - 2016-07-22 02:04 -0700
                Re: how to compute one and filter it immediately rather than calculated all and then filter the whole meInvent bbird <jobmattcon@gmail.com> - 2016-07-22 04:33 -0700
                Re: how to compute one and filter it immediately rather than calculated all and then filter the whole meInvent bbird <jobmattcon@gmail.com> - 2016-07-23 02:36 -0700
            Re: how to compute one and filter it immediately rather than calculated all and then filter the whole meInvent bbird <jobmattcon@gmail.com> - 2016-07-25 00:55 -0700

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