[Haskell-beginners] Improve my lambda expressions

[PATRICK BROWNE]

Thanks for all your help.
I was unaware that there was a relation between let/where and lambdas.
Here is my effort to use a single lamda
md3 n = (\n -> (dist  (Point (4.0 + 0.5 * n) (4.0 - 0.5 * n)) (Point (n *
1.0) ( n * (-1.0))))) n

I imagine that this function could be written without lambdas, let, or
where.
Is it generally true the all/most functions could be written without
lambdas, let, or where?

Thanks,
Pat



On 26 June 2017 at 12:04, Frerich Raabe <raabe at froglogic.com> wrote:

> On 2017-06-26 11:38, PATRICK BROWNE wrote:
>
>> 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.
>>
>
> [..]
>
> -- 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
>>
>
> Maybe you could reduce the number of lambda expressions by extracting
> common logic. It seems to me that 'mp1' and 'mp2' are moved versions of the
> same point (2,2) except that they apply different functions to the
> coordinates. These functions follow a pattern (a factor is applied to the
> component and then an 'offset' is added).
>
> For instance, it might be worthwhile to define
>
>   movePoint :: Float -> Float -> Point -> Point
>   movePoint offset factor (Point x y) = Point (offset + factor * x)
> (offset - factor * y)
>
> ...such that you could then define
>
> md = let p = Point 2 2 in dist (movePoint 4 0.5 p) (movePoint 0 1 p)
>
> --
> Frerich Raabe - raabe at froglogic.com
> www.froglogic.com - Multi-Platform GUI Testing
>

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