[Haskell-beginners] Improve my lambda expressions

[Silent Leaf]

yes you can, it's actually better to encapsulate all "lambdas" and
intermediate values inside one function, provided you never use them
anywhere else.
i advise using "where" as such:

md n = dist (Point (fx n) (fy n)) (Point (gx n) (gy n))
  where fx n = 4.0 + 0.5 * n
        fy n = 4.0 - 0.5 * n
        gx n = n * 1.0
        gy n = n * (-1.0)
-- then you can call "foo = md 2"

it's more idiomatic to use where, i believe. however you can also choose to
define the helpers before the expression defining the function, using "let
... in" like that:

md n = let fx n = 4.0 + 0.5 * n
           fy n = 4.0 - 0.5 * n
           gx n = n * 1.0
           gy n = n * (-1.0)
       in dist (Point (fx n) (fy n)) (Point (gx n) (gy n))

be careful about the alignment. both ways are (in this example) strictly
equivalent, it all depends on circumstances and taste.

mind you if you don't want a function to be reused, merely one single value
foo = md 2, you can always just write directly:
foo = dist (Point (fx 2) (fy 2)) (Point (gx 2) (gy 2))
  where fx n = 4.0 + 0.5 * n
        fy n = 4.0 - 0.5 * n
        gx n = n * 1.0
        gy n = n * (-1.0)

2017-06-26 11:38 GMT+02:00 PATRICK BROWNE <patrick.browne at dit.ie>:

> The code below provides a distance function that works for points and
> moving point.
> I am happy with the result, but I have a problem with the lambda
> expressions.
> Instead of supplying an argument to each 'mpXX', is it possible to have a
> single lambda argument on the entire final 'md' function? (e.g. 'md 2')
> Thanks in advance,
> Pat
>
> data Point a = Point { abscissa :: a, ordinate :: a } deriving Show
> dist a b = sqrt (((abscissa a) - (abscissa b))^2 + ((ordinate a) -
> (ordinate b))^2)
>
> -- Sttic points
> p1, p2 :: Point Float
> p1 = Point 0.0 0.0
> p2 = Point 4.0 4.0
> d = dist p1 p2
>
>
> -- Moving points
> mp1, mp2 :: Point Float
> mp1x = (\t -> 4.0 + 0.5 * t)
> mp1y = (\t -> 4.0 - 0.5 * t)
> mp1 = Point (mp1x 2) (mp1y  2)
> mp2x  = (\t -> 0.0 + 1.0 * t)
> mp2y  = (\t -> 0.0 - 1.0 * t)
> mp2 = Point (mp2x 2) (mp2y 2)
> md = dist mp1 mp2
>
> This email originated from DIT. If you received this email in error,
> please delete it from your system. Please note that if you are not the
> named addressee, disclosing, copying, distributing or taking any action
> based on the contents of this email or attachments is prohibited.
> www.dit.ie
>
> Is ó ITBÁC a tháinig an ríomhphost seo. Má fuair tú an ríomhphost seo trí
> earráid, scrios de do chóras é le do thoil. Tabhair ar aird, mura tú an
> seolaí ainmnithe, go bhfuil dianchosc ar aon nochtadh, aon chóipeáil, aon
> dáileadh nó ar aon ghníomh a dhéanfar bunaithe ar an ábhar atá sa
> ríomhphost nó sna hiatáin seo. www.dit.ie
>
> Tá ITBÁC ag aistriú go Gráinseach Ghormáin – DIT is on the move to
> Grangegorman <http://www.dit.ie/grangegorman>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/beginners/attachments/20170626/72891fac/attachment-0001.html>