Type-safe named fields in haskell ADTs -

Type-safe named fields in haskell ADTs -

there's mutual problem easy haskell. code-snippet describes this:

data jobdescription = jobone { n :: int } | jobtwo | jobthree { n :: int } deriving (show, eq) taskoneworker :: jobdescription -> io () taskoneworker t = putstrln $ "n: " ++ (show $ n t) main :: io () main = -- runs ok: taskoneworker (jobone 10) -- fails @ runtime: -- taskoneworker jobtwo -- works, didn't want to: -- taskoneworker (jobthree 10)

i described problem , it's possible solutions in article: https://www.fpcomplete.com/user/k_bx/playing-with-datakinds

here, on stackoverflow, i'd inquire -- best solutions problem, using tools , documentation?

i suggest using prism. prism _jobone represents int value lives within jobone constructor. in taskoneworker using ^?. either value t indeed jobone constructor , argument homecoming in just, or not jobone constructor , nothing.

{-# language templatehaskell #-} import control.lens info jobdescription = jobone { n :: int } | jobtwo | jobthree { n :: int } deriving (show, eq) $(makeprisms ''jobdescription) taskoneworker :: jobdescription -> io () taskoneworker t = case t ^? _jobone of n -> putstrln $ "n: " ++ show n nil -> putstrln "not jobone" -- or 'return ()' if want ignore these cases main :: io () main = taskoneworker (jobone 10) taskoneworker jobtwo taskoneworker (jobthree 10) -- *main> main -- n: 10 -- not jobone -- not jobone

haskell