Example.hs

-- V a b == a ± b
--
-- For example:
-- V 100 2 = 100 mm ± 2mm
data V = V
  { -- Middle value
    value :: Double,
    -- Must be positive
    tollerance :: Double
  }
  deriving (Show)

maximal :: V -> Double
maximal (V v t) = v + t

minimal :: V -> Double
minimal (V v t) = v - t

addV :: V -> V -> V
addV (V v1 t1) (V v2 t2) = V (v1 + v2) (t1 + t2)

-- FIXME: This doesn't work for negative numbers
maxV :: V -> V -> V
maxV va@(V v1 t1) vb@(V v2 t2) =
  let v = max v1 v2
      mx = max (maximal va) (maximal vb)
   in V v (mx - v)

data Fits
  = DefinitelyNot
  | Perhaps
  | Definitely
  deriving (Show)

fitsIn :: V -> V -> Fits
fitsIn v1 v2 =
  if maximal v1 < minimal v2
    then Definitely
    else
      if minimal v1 > maximal v2
        then DefinitelyNot
        else Perhaps

data Box = Box {height :: V, width :: V}
  deriving (Show)

box1 :: Box
box1 = Box (V 50 2) (V 50 2)

box2 :: Box
box2 = box1

availableWidth :: V
availableWidth = V 100 2

nextTo :: Box -> Box -> Box
nextTo (Box h1 w1) (Box h2 w2) = Box (maxV h1 h2) (addV w1 w2)

fitsInTruck :: Box -> V -> Fits
fitsInTruck (Box _ w) v = w `fitsIn` v