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@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@froglogic.com
www.froglogic.com - Multi-Platform GUI Testing

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