Is there a better/simpler way to achieve this?

[Laurence Roberts]

I'm trying to convert a list of coordinates into a series of points for svg lines, so as below:

-- from this
[ { lat = 0, lon = 0 }

, { lat = 1, lon = 1 }

, { lat = 2, lon = 2 }

, { lat = 3, lon = 3 }

]

-- to this

[ { x1 = 0, y1 = 0, x2 = 1, y2 = 1 }

, { x1 = 1, y1 = 1, x2 = 2, y2 = 2 }

, { x1 = 2, y1 = 2, x2 = 3, y2 = 3 }

]

This is how I've achieved it so far - http://share-elm.com/sprout/55ba9035e4b06aacf0e8b7ee

There seems to be some redundancy in the way I've got it working, particularly with the need to reverse and get the tail. Can anyone think of a better way of doing this? Thanks :)

[Hassan Hayat]

How about this?
http://share-elm.com/sprout/55ba9a0ee4b06aacf0e8b80a

The trick is to pluck out the tail and use List.map2

[Laurence Roberts]

That's fantastic, much cleaner!

I haven't seen cons used inside a case pattern like that before. I'm not sure I entirely understand what's going on there. It is like saying "if I am able to add an element to this list then run this function", but without actually doing the cons, just checking it's possible?

[xash]

Without pattern matching it could be http://share-elm.com/sprout/55ba9d5ce4b06aacf0e8b829

[Laurence Roberts]

Ah interesting, that seems a bit more straightforward

[Hassan Hayat]

Ok, xash beat me to it, but here's my version without the pattern matching.

http://share-elm.com/sprout/55ba9ec7e4b06aacf0e8b837

The idea is that I define toLine on individual coords to give me a line

toLine : Coord -> Coord -> Line

Then, given a list of coords

coords = [ c1, c2, c3, ... ]

I create a new list without the head

coordsTail = [c2, c3, c4, ... ]

And now, what I want is to apply "toLine" on c1 with c2 and c2 with c3 as so 

[ c1 , c2 , c3 , ... ]
  |     |    |
  |     |    |
  v     v    v
[ c2 , c3 , c4 , ... ]

And this is done via List.map2

[Laurence Roberts]

Brilliant, thanks for the detailed explanation.