How to get a variables only defined as an argument in another function in Haskell

Question

I have

cubicQ a b c = ((3*a*c) - b**2)/(9 * (a**2))

I need to get the value of "a" out of that so I can use it in another function, without having to pass it as an argument in that other function. The function I need to use it in is below:

cubicRealSolution q r s t = if p < 0 then error "NaN" else (s + t) - ((b)/(3*(a)))

This looks like an (almost) complete duplicate of the question you asked two minutes ago stackoverflow.com/questions/52484370/… — willeM_ Van Onsem
– willeM_ Van Onsem, Commented Sep 24, 2018 at 17:22
What do you think the values of a and b would be? You haven't called cubicQ at all, and whatever values you would use in such a call are the values you would use in cubicRealSolution in the first place. — chepner
– chepner, Commented Sep 24, 2018 at 17:25
@chepner: in the linked question, the OP defines a q, etc., but does not really makes a "function application". — willeM_ Van Onsem
– willeM_ Van Onsem, Commented Sep 24, 2018 at 17:31

Daniel Wagner · Accepted Answer · 2018-09-24 17:33:13Z

5

Sorry, it's not possible. If you want a function to have access to a value, you must pass that value to the function (and it must be prepared to accept it).

answered Sep 24, 2018 at 17:33

Daniel Wagner

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1 Comment

Elmex80s Over a year ago

You can wrap a function inside another one using the where keyword. The wrapped function will have access to all the parameters from the function it is wrapped in. So not impossible although it depends on the situation.

Elmex80s · Accepted Answer · 2018-09-24 22:37:25Z

0

Altough this won't work we'll present you this to show the idea. In Haskell you can write

cubicQ a b c = ((3*a*c) - b**2)/(9 * (a**2))

    where

    cubicRealSolution q r s t = if p < 0 then error "NaN" else (s + t) - ((b)/(3*(a)))

The trick is to use the where keyword. Because cubicRealSolution is wrapped inside cubicQ it can have access to its arguments a, b and c without the need to pass them.

answered Sep 24, 2018 at 22:37

Elmex80s

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