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Kevin
Kevin

Reputation: 145

Foldl with lambda expresion

Hi I'm trying to sum a list of tuples into a tuple with the foldl function, I tryed it with using as parameter a lambda expresion but it's giving out a wrong value here the code:

data Point = Point {x,y :: Float}
sumPoint :: [Point] -> (Float,Float)
sumPoint xs = foldl (\(a,b) x-> (0+a,0+b)) (0.0,0.0) xs

It should come out sumPoint [Point 2 4, Point 1 2, Point (-1) (-2)] = (2.0,4.0) But im getting (0.0,0.0) How is this making any sense?

Upvotes: 1

Views: 244

Answers (2)

Redu
Redu

Reputation: 26161

To be a little structural you better define operations among Point type values and then convert the Point type to Tuple wherever needed. Otherwise you may directly use Tuple and discard the Point type.

data Point = Point {x,y :: Float} deriving Show

toTuple :: Point -> (Float, Float)
toTuple p = (x p, y p)

addPts :: Point -> Point -> Point
addPts p q = Point (x p + x q) (y p + y q)

sumPts :: [Point] -> Point
sumPts = foldl addPts (Point 0 0)

So what you need is toTuple . sumPts function.

*Main> :t toTuple . sumPts
toTuple . sumPts :: [Point] -> (Float, Float)

Upvotes: 3

Kevin
Kevin

Reputation: 145

I changed it to 

sumPoint xs = foldl (\(a,b) (Point x y)-> (x+a,y+b)) (0.0,0.0) xs

The problem was I was ignoring the x and at 0+a is nothing happening.

Upvotes: 2

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