haskell - promoted datatypes and class instances -

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promoted datatypes have fixed number of types members of promoted data kind . in closed world make sense support calling function on typeclass without dictionary explicitly in scope?

{-# language datakinds #-} {-# language polykinds #-}  data datatype = constructor  data datatypeproxy (e :: datatype) = datatypeproxy  class class (e :: datatype)   classfunction :: datatypeproxy e -> io ()  -- instance can created instance class 'constructor   classfunction _ = return ()  -- adding class constraint fixes build break -- disp :: class e => datatypeproxy e -> io () disp :: datatypeproxy e -> io () disp = classfunction  main :: io () main = disp (datatypeproxy :: datatypeproxy 'constructor)

this contrived example doesn't work in ghc head. it's not @ surprising seems datakind extension might make possible.

test.hs:18:8:     no instance (class e) arising use of `classfunction'     possible fix:       add (class e) context of         type signature disp :: datatypeproxy e -> io ()     in expression: classfunction     in equation `disp': disp = classfunction

no. doing allowing mean phantom data types need "tagged" type information @ runtime, , produce ambiguity. 

data datatype = constructor | somethingelse  data datatypeproxy (e :: datatype) = datatypeproxy  ... instance class 'somethingelse    classfunction _ = putstrln "hello world"  instance class 'constructor    classfunction _ = return ()  disp :: datatypeproxy e -> io () disp = classfunction  main = disp datatypeproxy

what should program do? should not compile? if not, adding constructor type, took program compile, , produced 1 not. if should not compile has 2 valid behaviors. 

main = disp (datatypeproxy :: datatypeproxy 'constructor)

has 1 possible interpretation...but requires dispatch on phantom type. is,

main = disp (datatypeproxy :: datatypeproxy 'somethingelse)

is identical program @ term level, has different behavior. breaks nice properties parametrically. typeclass based dispatching solution this, since instances in scope effect program behavior in predictable (and semantically specified) way.


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