[Haskell-cafe] Define combination of type classes?

Gerrit van den Geest g.vandengeest at students.uu.nl
Fri Mar 24 03:09:58 EST 2006

Another option is to just add the instance:

 > instance FooBar Char

instead of

 > instance (Foo a, Bar a) => FooBar a

Now you don't need any extensions, the disadvantage is that you have to 
add an instance for each type...

There has also been an proposal for type class synonyms: 

Maybe you like it,


Sean Seefried wrote:

> On 24/03/2006, at 12:45 PM, Fritz Ruehr wrote:
>> What is the easiest way to name a combination of type classes,  i.e., 
>> to abbreviate the fact that a certain type is an instance of  several 
>> classes simultaneously? I have a vague sense that this is  do-able, 
>> but that I am messing up by trying to use an empty class  body as below.
>> So in the code below, I try to use FooBar to abbreviate the  
>> conjunction of Foo and Bar. But while f (which uses a FooBar  
>> constraint) has a valid definition, it can't be used. On the other  
>> hand, g (which uses the long-winded constraint), is both a valid  
>> defined and useable.
>> (In a real example, imagine that FooBar names a conjunction of a  
>> half dozen things, so that the g-like form really is onerous,  
>> whereas the f-like form would be sweet and tidy :) .)
> Hi Fritz!
> You only need to do a couple of things to get this working. Add an  
> instance declaration:
> instance (Foo a, Bar a) => FooBar a
> But for this to work you need to allow undecidable instances (and - 
> fglasgow-exts).
> To have this type class synonym trick work you need both the class  
> and instance declaration:
> class    (Foo a, Bar a) => FooBar a
> instance (Foo a, Bar a) => FooBar a
> The first ensures that members of class FooBar will inherit the  
> methods of classes Foo and Bar. The second ensures that if there is a  
> Foo and a Bar instance then there will be a FooBar instance. You were  
> lacking this in your code hence the error message:
> > f 'a'
> No instance for (FooBar Char)
>       arising from use of `f' at <interactive>:1:0
>     Probable fix: add an instance declaration for (FooBar Char)
>     In the definition of `it': it = f 'a'
> This is a neat trick. I've also used it to reduce onerous contexts.
> Sean
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