haskell - Anonymous Type Functions -


this followup previous question: type-level map datakinds, starting 2 answers received.

my goal take hlist of arbitrary types , turn list of related/derived types

type family typemap (a :: * -> *) (xs :: [*]) :: [*] type instance typemap t '[] = '[] type instance typemap t (x ': xs) = t x ': typemap t xs  data hlist :: [*] -> *           hnil :: hlist '[]           hcons :: -> hlist -> hlist (a ': as)

when tried few types, ran problem. type-function " argument typemap has take hlist element type last argument , return new type. works okay sometimes:

test :: hlist rqs -> hlist (typemap ((,) int) rqs) test hnil = hnil test (hcons x xs) = hcons (3,x) $ test xs

but if wanted switch order of tuple in definition of test? first attempt define type synonym:

type revinttup b = (b,int)  test2 :: hlist rqs -> hlist (typemap revinttup rqs) test2 hnil = hnil test2 (hcons x xs) = hcons (x,3) $ test2 xs

but of course, you can't partially apply type synonyms, trick. there (other) way achieve this?

you should able write fliptypemap... that's not composable. better choice here might type-level version of map ($ 2) (map (/) [1,2,3]) instead of map (flip (/) 2) [1,2,3] taking advantage of -xpolykinds:

type family typemap (a :: j -> k) (xs :: [j]) :: [k] type instance typemap t '[] = '[] type instance typemap t (x ': xs) = t x ': typemap t xs  type family aplist (xs :: [j -> k]) (a :: j) :: [k] type instance aplist '[] t = '[] type instance aplist (x ': xs) t = x t ': aplist xs t  test2 :: hlist rqs -> hlist (typemap (,) rqs `aplist` int) test2 hnil = hnil test2 (hcons x xs) = hcons (x,3) $ test2 xs

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