list,haskell,functional-programming,idiomatic , Idiomatic list construction


Idiomatic list construction

Question:

Tag: list,haskell,functional-programming,idiomatic

I'm very new to Haskell and functional programming in general, so I don't really know how to make this code idiomatic:

type Coord = Double
data Point = Point Coord Coord Coord deriving Show

type Polyline = [Point]

-- Add a point to a polyline
addPoint :: Polyline -> Point -> Polyline
addPoint line p = p:line

line :: Polyline
line = []

constructLine :: Polyline -> Polyline
constructLine line =
    let 
        p1 = Point 2 4 87
        p2 = Point 3 7 2
        p3 = Point 23 4 8
    in addPoint (addPoint (addPoint line p1) p2) p3

main :: IO() 
main = do
    putStrLn ( show (constructLine line))

My problem is in the constructLine function. If I want to add a lot of points the nested addPoint functions are going to be a problem. How can I factor this? And do you see other things that could be improved?


Answer:

The multiple call to addPoints could be replaced by a fold. As suggested in a comment, reversing your addPoint function would make things easier:

addPoint' :: Point -> Polyline ->  Polyline
addPoint' p line = p:line

So then your constructLine function could build a temporary list of the points to add add use a fold:

constructLine :: Polyline -> Polyline
constructLine line =
    let
        p1 = Point 2 4 87
        p2 = Point 3 7 2
        p3 = Point 23 4 8
    in foldr addPoint' line [p3,p2,p1]

This does not break the encapsulation (you can replace your implementation of Polyline by something else than a list of Point) and uses the new points in the order they're going to end up at (p3 in front of p2, etc.)


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