[Haskell-beginners] Prime number test performance on negative integers

[Francesco Ariis]

Hello Pratima,

Il 11 dicembre 2023 alle 19:24 Folsk Pratima ha scritto:
> Please explain why this fails for negative numbers. By fails I mean it
> starts eating RAM infinitely until either me or OOM kills it. If you
> replace `m1` definition with the one commented out, the code will fail
> for positive integers as well, which is very frustrating.

I have replaced m1 definition with the commented one, and it works on
my machine.

    f at x270:/tmp/prova$ ./prime -4
    -4 is not primal because divisible by 2

How are you invoking the program?

A few additional notes (run `hlint` for more)

> main =
>     getArgs >>= \argv ->

I don’t mind `>>=` but with `do` notation and `traceM/traceShowM` it
is easier to debug your programs.

>             --m1 =
>             --    map
>             --        (\x ->
>             --             if x < 0
>             --                 then -x
>             --                 else x)
>             --        m0

More compact: `m1 = map abs m0`

>             msg (x, (y0, y1)) = x ++ " is " ++ f y0 ++ " because " ++ y1

Ancillary functions like this one can go in the `where` section.
Ciao
—F