-----------------------------------------------------------------------
--	Shape example, from Haskell craft, Ch 14
--	Added triangle constructor to Shape1
-- 
-- 	21 June 2001
-----------------------------------------------------------------------

module Shape2 where


-- A shape in a simple geometrical program is either a circle or a
-- rectangle. These alternatives are given by the type

data Shape = Circle Float |
             Rectangle Float Float |
	     Triangle Float Float Float			--<<<<

shape1 = Circle 3.0
shape2 = Rectangle 45.9 87.6

-- Pattern matching allows us to define functions by cases, as in,

isRound :: Shape -> Bool
isRound (Circle _)      = True
isRound (Rectangle _ _) = False
isRound (Triangle _ _ _) = False			--<<<<

-- and also lets us use the components of the elements:

area :: Shape -> Float
area (Circle r)      = pi*r*r
area (Rectangle h w) = h*w
area (Triangle x y z) = sqrt(s*(s-x)*(s-y)*(s-z))	--<<<<
			where s = (x+y+z)/2		--<<<<

perimeter :: Shape -> Float
perimeter (Circle r)      = 2*pi*r
perimeter (Rectangle h w) = 2*(h+w)
perimeter (Triangle x y z) = x+y+z			--<<<<