Re: [Haskell-cafe] "Generating" type synonyms without Template Haskell

On Fri, Jun 27, 2014 at 10:30:45PM +0200, Niklas Haas wrote:

On Fri, 27 Jun 2014 17:52:39 +0100, Tom Ellis wrote:

...

I'd like to know how to conveniently generate specialisations of a product type without resorting to Template Haskell.

Concretely, I have a type like

data MyProduct a b c = MyProduct { foo :: a , bar :: b , baz :: c }

and I use it in type signatures in various different specialisations. Generally the "base" type of each component stays fixed but it is wrapped in zero or more type constructors. For example

a. MyProduct Int Bool String

b. MyProduct (Maybe Int) (Maybe Bool) (Maybe String)

c. MyProduct [IO Int] [IO Bool] [IO String]

d. MyProduct (P Int) (P Bool) (P String)

Using LiberalTypeSynonyms and your original MyProduct, we can define

type Example f = MyProduct (f Int) (f Bool) (f String)

type Id x = x type ListIO x = [IO x]

type A = Example Id type B = Example Maybe type C = Example ListIO type D = Example P

Oh, that looks like exactly what I wanted! I didn't know about LiberalTypeSynonyms. Thanks a lot, Tom