[Haskell-cafe] Data constructor synonyms
Hello! Assume I have a type MyType with the constructor GeneralConstructor: data MyType = GeneralConstructor [Double] but I also want to have a separate name for special case of this constructor: SpecialConstructor Double so SpecialConstructor a = GeneralConstructor (a:[]) that is SpecialConstructor 5 was exactly the same as GeneralConstructor [5]. And for example instead of writing GeneralConstructor [0] I would like to use constructor Zero. It's all just for convenience. How can I achieve this? Well, of course I always can use sed to replace SpecialConstructor 5 with GeneralConstructor [5] in program sources, but it's not convenient. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Data constructor synonyms
Remember that constructors are functions, except that you can't pattern match against them. data MyType = GeneralConstructor [Double] -- GeneralConstructor :: [Double] - MyType Note the lower case character, just a plain function: specialConstructor :: Double - MyType specialConstructor a = GeneralConstructor (a:[]) zero :: MyType zero = GeneralConstructor [0] The downside is that you can't pattern-match against these functions. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Data constructor synonyms
18.03.2011, 14:22, Roel van Dijk vandijk.r...@gmail.com;: Remember that constructors are functions, except that you can't pattern match against them. .. The downside is that you can't pattern-match against these functions. The thing is that I need pattern matching, just functions won't do. Anyway, a new question arose. If I have already declared a type, can I add new constructors to it from other modules? Maybe there are some GHC extensions to solve both these problems. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Data constructor synonyms
On 18 March 2011 13:31, Grigory Sarnitskiy sargrig...@ya.ru wrote: Anyway, a new question arose. If I have already declared a type, can I add new constructors to it from other modules? Maybe there are some GHC extensions to solve both these problems. no can do. There are ways to encode extensible types (e.g the finally tagless sytle), but on balance you are better to design extensibility for functions - easy to add more functions - than make your code much more complicated so it can be extensible for types. Parser combinators and pretty print combinators are great and largely simple examples of extensibility with functions. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Data constructor synonyms
Remember that constructors are functions, except that you can't pattern match against them. .. The downside is that you can't pattern-match against these functions. The thing is that I need pattern matching, just functions won't do. It's only a preprocessor, but Conor's she allows pattern synonyms: http://personal.cis.strath.ac.uk/~conor/pub/she Anyway, a new question arose. If I have already declared a type, can I add new constructors to it from other modules? Again, not within Haskell itself. she also has a feature that allows something like this. There are various other techniques or proposals. For example: http://www.cs.ru.nl/~wouters/Publications/DataTypesALaCarte.pdf http://people.cs.uu.nl/andres/OpenDatatypes.html Cheers, Andres ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Data constructor synonyms
You are better to use simple typeclasses. It depends on what you are trying to do, but when I want an open type, I use classes + type families. 2011/3/18 Stephen Tetley stephen.tet...@gmail.com On 18 March 2011 13:31, Grigory Sarnitskiy sargrig...@ya.ru wrote: Anyway, a new question arose. If I have already declared a type, can I add new constructors to it from other modules? Maybe there are some GHC extensions to solve both these problems. no can do. There are ways to encode extensible types (e.g the finally tagless sytle), but on balance you are better to design extensibility for functions - easy to add more functions - than make your code much more complicated so it can be extensible for types. Parser combinators and pretty print combinators are great and largely simple examples of extensibility with functions. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
