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.