Re: [Haskell-cafe] Avoiding name collisions by using value spaces instead of modules

----- Original Message ----- From: "Cale Gibbard" To: "Brian Hulley" Cc: "Daniel Fischer" ; "Haskell-cafe" Sent: Sunday, January 08, 2006 5:54 PM Subject: Re: [Haskell-cafe] Avoiding name collisions by using value spaces instead of modules

On 08/01/06, Brian Hulley wrote:

...

For example, suppose I'm writing a module M that deals with grammar, where the elements in a grammar rule are parameterised so that rules can be written using strings but processed as if we'd used ints instead:

data Element a = Terminal a | Nonterminal a | Action a

data Rule a = Rule (Element a) [[Element a]]

Now I want to convert elements and rules from a to Int, so at the moment I have to write:

convertElement :: Element a -> CM (Element Int) ...

convertRule :: Rule a -> CM (Rule Int)

for some appropriate monad CM. Whereas I would have much preferred to use just the word "convert" in both cases. It is tremendously annoying to have to suffix everything with the type name.

This is what typeclasses are for.

class Convert c where convert :: c a -> CM (c Int)

Type classes just seem overkill for this kind of thing. All I want is compile time resolution of an overloaded identifier, whereas type classes give all the machinery that would be needed if I wanted runtime ad-hoc polymorphism, with all the attendant verbosity, just so that the compiler can then optimize out the runtime polymorphism behind the scenes for cases like the example above. After all, I just want to write two very simple functions, so the effort to factor into a type class + two instances, also having to include the Convert c in the context whenever I call one of them just seems really painful. Also, the word "Convert" is now used up as well... Also, when I'm just writing code in an exploratory way, I don't know in advance what the common things are that could be factored out into type classes (except perhaps in very simple examples like that above), so while I'm writing the code for the first time there is still a problem trying to think up different names. Regards, Brian.