How do I define a data type that only accepts numbers?

Question

I am attempting to create a data type, Point, that takes three numbers for its constructor. Initially, I had written

data Point = Point Double D         


        

Answer 1

You can use the Num typeclass to enforce Num constraint on your datatype. The usual syntax would be something like:

data MyTypeClass a => MyDataType a = MyDataTypeConstructor1 a|MyDataTypeConstructor2 a a|{- and so on... -}

In your case, you can do

data Num a => Point a = Point a a a 

Read more about data type specifications and LYAH. Real World Haskell also mentions this.

EDIT

As mentioned by shachaf, this is not quite valid haskell2010, though the specification mentions it. I should also note that this form is seldom used and the preferred way to enforce these constraints through functions than through typeclasses/datatypes, since they introduce an additional dependency on types.