[Haskell-cafe] labels in HList

roger peppe rogpeppe at gmail.com
Wed Oct 22 06:17:05 EDT 2008


i'm trying to get labelled records working with the current version of HList.

i've got code that looks like:

>{-# language EmptyDataDecls #-}
>module Tst where
>	import Data.HList
>	data Foo;    foo     = proxy::Proxy Foo
>	data Bar;   bar    = proxy::Proxy Bar
>	rec1 =
>		foo .=. 1 .*.
>		bar .=. "hello" .*.
>		emptyRecord

which gives me the error:

    No instance for (HEq (Proxy Foo) (Proxy Bar) HFalse)
      arising from a use of `.*.' at Tst4.hs:(8,2)-(10,12)
    Possible fix:
      add an instance declaration for
      (HEq (Proxy Foo) (Proxy Bar) HFalse)
    In the expression: foo .=. 1 .*. bar .=. "hello" .*. emptyRecord
    In the definition of `rec1':
        rec1 = foo .=. 1 .*. bar .=. "hello" .*. emptyRecord

some discussion on #haskell suggesting importing Label4 and TypeEqGeneric1
but a) that's not possible because both are hidden inside the HList package
and b) even when i get around that restriction, i still get a "No instance
for (TypeCast HFalse HFalse)" error.


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