Introducing shape data type in Haskell

Question

I saw this exercise in a book and I am trying to do it but I have a question.

What I am trying to do is define a data type Shape suitable for representing triangles, squares and circles, and then define a function area :: Shape -> Float which returns the area of a given Shape.

My code right now:

data Shape = Triangle Float Float Float 
            | Square Float | Circle Float

area :: Shape -> Float
area (Triangle a b c) = sqrt (s*(s-a)*(s-b)*(s-c))
        where
            s = (a+b+c)/2
area Square d = d*d
area Circle r = pi * r^2

The error I get:

Haskell.hs:4:1:
Equations for `area' have different numbers of arguments
  Haskell.hs:(4,1)-(6,37)
  Haskell.hs:7:1-19
Failed, modules loaded: none.

I saw the solution is this :

data Shape = Triangle Float Float Float 
   | Square Float | Circle Float 
area :: Shape -> Float 
area ( Triangle a b c ) 
   = triarea a b c 
area ( Square d ) 
   = d * d 
area ( Circle r ) 
   = pi * r^2 

triarea a b c 
   = sqrt( s * (s-a) * (s-b) * (s-c) ) 
     where 
     s = (a+b+c)/2 

How is this different from my implementation?? What does my error mean?

Thanks :)

Answer 1

You need to surround Square d and Circle r in parentheses. Otherwise they'll be parsed as two arguments, not one.

See, for example, this answer: "Function application binds tighter than any infix operator. Burn this into your brain in letters of fire." (That's not completely pertinent here; rather, function application is left-associative, even on the lhs of a definition.)