Hi Henning, thanks for the code review -- reason that I don't use the type declaration a lot -- It causes trouble , because I don't yet fully understand it. When I declare what I think is right is fails - see Message at bottom -- so what's wrong ? By the way I just used lists so far - no arrays -- Why is !! inefficient ? - what is better - Cheers Phil 1 import Data.List 2 3 -- Input Data 4 xi :: [Double] 5 xi = [0 .. 10] 6 yi :: [Double] 7 yi = [2, 3, 5, 6, 7, 8, 9, 10 , 9, 8, 7] 8 x = 11 :: Double 9 10 -- Functions 11 limIndex :: [a] -> Int -> Int 12 limIndex xi idx 13 | idx < 0 = 0 14 | idx > (length xi)-2 = (length xi)-2 15 | otherwise = idx 16 17 getIndex :: [a] -> Double -> Int 18 getIndex xi x = limIndex xi (maybe (length xi) id (findIndex (>x) xi)-1) 19 20 getPnts :: [a] -> [a] -> Int -> [a] 21 getPnts xi yi idx = [xi !! idx, xi !! (idx+1), yi !! idx, yi !! (idx+1)] 22 23 interp :: [a] -> [a] -> Double -> Double 24 interp xi yi x = 25 let pts = getPnts xi yi (getIndex xi x) 26 in (pts!!3-pts!!2)/(pts!!1-pts!!0)*(x-pts!!0)+pts!!2 27 28 -- Calc 29 y = interp xi yi x 30 31 main = do 32 -- Output Data 33 print (y) === Compiler Error Message === interp_v4.hs:18:66: Couldn't match expected type `Double' against inferred type `a' `a' is a rigid type variable bound by the type signature for `getIndex' at interp_v4.hs:17:13 Expected type: [Double] Inferred type: [a] In the second argument of `findIndex', namely `xi' In the third argument of `maybe', namely `(findIndex (> x) xi)' interp_v4.hs:26:39: Couldn't match expected type `Double' against inferred type `a' `a' is a rigid type variable bound by the type signature for `interp' at interp_v4.hs:23:11 In the second argument of `(-)', namely `pts !! 0' In the second argument of `(*)', namely `(x - pts !! 0)' In the first argument of `(+)', namely `(pts !! 3 - pts !! 2) / (pts !! 1 - pts !! 0) * (x - pts !! 0)' -- View this message in context: http://haskell.1045720.n5.nabble.com/HMatrix-Vector-Matrix-interpolation-ala... Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.