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authorJoey Hess2015-01-24 16:54:49 -0400
committerJoey Hess2015-01-24 16:54:49 -0400
commit45c94ffdd79b4f8134ef651b08fedb57b37448e6 (patch)
treef6f156c9281d91654d1a7675841ac1bf3adab715 /src/wrapper.hs
parent414ee7eee60300eb7f7c49e4890b056d19b3c59b (diff)
moving to using the GADT
The problem this exposes has to do with requires. As implemented, requires yields either a Property HasInfo or a Property NoInfo depending on its inputs. That works. But look what happens when it's used: *Propellor.Types> let foo = IProperty "foo" (return NoChange) mempty mempty *Propellor.Types> let bar = IProperty "bar" (return NoChange) mempty mempty *Propellor.Types> foo `requires` bar <interactive>:17:5: No instance for (Requires (Property HasInfo) (Property HasInfo) r0) arising from a use of `requires' The type variable `r0' is ambiguous Possible fix: add a type signature that fixes these type variable(s) Note: there is a potential instance available: instance Requires (Property HasInfo) (Property HasInfo) (Property HasInfo) -- Defined at Propellor/Types.hs:167:10 Possible fix: add an instance declaration for (Requires (Property HasInfo) (Property HasInfo) r0) In the expression: foo `requires` bar In an equation for `it': it = foo `requires` bar This can be avoided by specifying the result type: *Propellor.Types> (foo `requires` bar) :: Property HasInfo property "foo" But then when multiple `requires` are given, the result type has to be given each time: *Propellor.Types> (foo `requires` bar `requires` bar) :: Property HasInfo <interactive>:22:6: No instance for (Requires (Property HasInfo) (Property HasInfo) x0) arising from a use of `requires' The type variable `x0' is ambiguous Possible fix: add a type signature that fixes these type variable(s) Note: there is a potential instance available: instance Requires (Property HasInfo) (Property HasInfo) (Property HasInfo) -- Defined at Propellor/Types.hs:167:10 Possible fix: add an instance declaration for (Requires (Property HasInfo) (Property HasInfo) x0) In the first argument of `requires', namely `foo `requires` bar' In the expression: (foo `requires` bar `requires` bar) :: Property HasInfo In an equation for `it': it = (foo `requires` bar `requires` bar) :: Property HasInfo <interactive>:22:21: No instance for (Requires x0 (Property HasInfo) (Property HasInfo)) arising from a use of `requires' The type variable `x0' is ambiguous Possible fix: add a type signature that fixes these type variable(s) Note: there are several potential instances: instance Requires (Property NoInfo) (Property HasInfo) (Property HasInfo) -- Defined at Propellor/Types.hs:175:10 instance Requires (Property HasInfo) (Property HasInfo) (Property HasInfo) -- Defined at Propellor/Types.hs:167:10 Possible fix: add an instance declaration for (Requires x0 (Property HasInfo) (Property HasInfo)) In the expression: (foo `requires` bar `requires` bar) :: Property HasInfo In an equation for `it': it = (foo `requires` bar `requires` bar) :: Property HasInfo *Propellor.Types> (((foo `requires` bar) :: Property HasInfo) `requires` bar) :: Property HasInfo property "foo" Yuggh!
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