Haskell - simple constructor comparison (?) function

Question

In my project I have created a data type, that can hold one of a few types of values:

data PhpValue = VoidValue | IntValue Integer | BoolValue Bool 

What I wanted to do now, is to have a simple way of checking if two values of the PhpValue type are of the same constructor (correct me if I'm confused with the terminology here, but basically what I want to check if both are, for example, are IntValue, without caring about the particular value).

Here is a function I wrote for that:

sameConstructor :: PhpValue -> PhpValue -> Bool sameConstructor VoidValue VoidValue = True sameConstructor (IntValue _) (IntValue _) = True sameConstructor (BoolValue _) (BoolValue _) = True sameConstructor _ _ = False 

This works as it should, but I don't really like it: if I add more constructors (like FloatValue Float) I am going to have to rewrite the function, and it will get bigger as my data definition gets bigger.

The Question: Is there a way of writing such a function, so that its implementation doesn't change when I add more constructors?

For the record: I don't want to change the data definition, I have enough Monads in the rest of my code as it is ;)

Answer 1

Take a look at Data.Data and its toConstr function. This returns a representation of the constructor which can be compared for equality.

With an extension (you can put {-# LANGUAGE DeriveDataTypeable #-} at the top of your module), you can have a Data instance derived for you automatically:

data PhpValue = VoidValue | IntValue Integer | BoolValue Bool                deriving (Typeable, Data) 

You should then be able to use the toConstr function to compare by constructor.

Now the following will be true:

toConstr (BoolValue True) == toConstr (BoolValue False) 

Using on from Data.Function you can now rewrite sameConstructor to:

sameConstructor = (==) `on` toConstr 

This is the same as

sameConstructor l r = toConstr l == toConstr r 

I think the version using on is easier to read at a glance.