On Fri, Mar 19, 2010 at 8:15 AM, Ozgur Akgun wrote:

Hi Cafe!

Disclaimer: I know what I'm going to ask is now available as a language feature normally.

Did you mean "not available"? I don't know of a Haskell language feature for this, so if you really did mean "now" then I'd love to learn about the feature you're thinking of. Only think I can think of is having the constructors, as seperate data

types, introducing new type classes to group every possible subset of [X,Y,Z] and [A,B,C,D] and use those type classes when defining the functions. But as you can imagine, this is not the only place I want to use this thing, and the code will start to look cryptic if I do so. I must add, I want to use this *disjunction of constructors* thingy in data type declerations as well.

It looks to me like you want dependent types. You might look at Agda. It's similar to Haskell but features dependent types and other interesting things. Jason

On Fri, Mar 19, 2010 at 03:15:02PM +0000, Ozgur Akgun wrote:

Hi Cafe!

Disclaimer: I know what I'm going to ask is now available as a language feature normally. And sorry for the subject of this message, I couldn't compe up with a good&descriptive subject.

Is there any way to limit a functions type, not by a data type but by a group of constructors of a data type? If not, what would be the *right* thing to do to achieve this level of type safety?

data DT1 = X | Y | Z data DT2 = A | B | C | D

func1 :: DT1 -> DT2 -- instead of this func1' :: (X|Y) -> (B|C) -- i want sth. like this. (| means or)

The standard way to do this is with GADTs and phantom type parameters. Something like this:

data XY data NotXY data BC data NotBC

data DT1 a where X :: DT1 XY Y :: DT1 XY Z :: DT1 NotXY

data DT2 a where A :: DT1 NotBC ... etc.

func1 :: DT1 XY -> DT2 BC ...

Every value of DT1 is tagged with an extra piece of information at the type level saying whether it is X or Y, or not. These extra types are known as "phantom type parameters" since nothing of those types actually show up in the values of the data type (indeed, XY, NotXY, and so on have no values) but they are only there to provide some extra information at the type level. Of course, the only problem with this approach is that if you want another function that only takes Y or Z, you are out of luck without some fancier type hackery. However, if you wish to know about a nice approach to said fancier type hackery, see Wouter Swierstra's paper "Data Types a la Carte" [1]. -Brent [1] http://www.cs.nott.ac.uk/~wss/Publications/DataTypesALaCarte.pdf

Ozgur Akgun wrote:

Is there any way to limit a functions type, not by a data type but by a group of constructors of a data type? If not, what would be the *right* thing to do to achieve this level of type safety?

data DT1 = X | Y | Z data DT2 = A | B | C | D

func1 :: DT1 -> DT2 -- instead of this func1' :: (X|Y) -> (B|C) -- i want sth. like this. (| means or)

OCaml has a feature called 'polymorphic variants' that allows exactly this. You may want to google 'polymorphic variants in haskell'.