[Haskell-cafe] Area from [(x,y)] using foldl

[michael rice]

Here's an (Fortran) algorithm for calculating an area, given  one dimensional
arrays of Xs and Ys. I wrote a recursive Haskell function that works, and one using
FOLDL that doesn't. Why would Haskell be "expecting" (t, t) out of ((*) (xold-x) (yold+y))?

Michael

====================

   AREA = 0.0
   XOLD = XVERT(NVERT)
   YOLD = YVERT(NVERT)
   DO 10 N = 1, NVERT
           X = XVERT(N)
           Y = YVERT(N)
           AREA = AREA + (XOLD - X)*(YOLD + Y)
           XOLD = X
           YOLD = Y
10 CONTINUE

   AREA = 0.5*AREA

====================

area :: [(Double,Double)] -> Double
area ps = abs $ (/2) $ area' (last ps) ps
            where area' _ [] = 0
                  area' (x0,y0) ((x,y):ps) = (x0-x)*(y0+y) + area' (x,y) ps



*Main> let p = [(0.0,0.0),(1.0,0.0),(1.0,1.0),(0.0,1.0),(0.0,0.0)]
*Main> area (last p) p
1.0
*Main> 

====================

area :: [(Double,Double)] -> Double
area p = foldl (\ (xold,yold) (x,y) -> ((*) (xold-x) (yold+y))) 0 ((last p):p)


Prelude> :l area
[1 of 1] Compiling Main             ( area.hs, interpreted )

area.hs:29:40:
    Occurs check: cannot construct the infinite type: t = (t, t)
      Expected type: (t, t)
      Inferred type: t
    In the expression: ((*) (xold - x) (yold + y))
    In the first argument of `foldl', namely
        `(\ (xold, yold) (x, y) -> ((*) (xold - x) (yold + y)))'
Failed, modules loaded: none.
Prelude> 




      
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