Can't make String an instance of a class in Haskell

Question

I'm playing around trying to understand classes in Haskell. I wrote a silly few lines of code to get the hang of it. I wrote a class called `Slang` that has one function. When I make Integer an instance of my class, it works fine. But when I make String an instance of my class it won't compile. I've been fidgeting with the program based on what the error output tells me but to no avail. I have idea why it work... Here is the code followed by the error: ``` module Practice where class Slang s where slangify :: s -> String instance Slang Integer where slangify int = "yo" instance Slang String where -- When I take this segment out, it works fine slangify str = "bro" ``` ERROR: ``` Prelude> :load Practice [1 of 1] Compiling Practice ( Practice.hs, interpreted ) Practice.hs:9:10: Illegal instance declaration for `Slang String' (All instance types must be of the form (T t1 ... tn) where T is not a synonym. Use -XTypeSynonymInstances if you want to disable this.) In the instance declaration…

Accepted Answer

The problem is that String is not a base type like Integer. What you are trying to do is actually

```
instance Slang [Char] where
    slangify str = "bro"
```

However, Haskell98 forbids this type of typeclass in order to keep things simple and to make it harder for people to write overlapping instances like

```
instance Slang [a] where
    -- Strings would also fit this definition.
    slangify list = "some list"
```

Anyway, as the error message suggests, you can get around this restriction by enabling the FlexibleInstances extension.

Can't make String an instance of a class in Haskell

Solution

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