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…

Accepted Answer

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.

Haskell - simple constructor comparison (?) function

Solution

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