Class Members with Type Variable Cause Compile Error in makeAction

[jeremyjh]

It seems only fixed types are supported by makeAction. If I change the type of readFile in the test case slightly, to say:

class MonadError e m => MonadFileSystem e m | m -> e where
  readFile :: Show a => a -> m String
  writeFile :: FilePath -> String -> m ()

Then the code produced by makeAction will not compile:

    • Could not deduce: a1 ~ a
      from the context: (r ~ [Char], Show a)
        bound by a pattern with constructor:
                   ReadFile :: forall a_XrHB.
                               Show a_XrHB =>
                               a_XrHB -> FileSystemAction String,
                 in an equation for ‘==’
        at /Users/jeremy/repos/haskell/monad-mock/test-suite/Control/Monad/MockSpec.hs:20:1-60
      ‘a1’ is a rigid type variable bound by
        a pattern with constructor:
          ReadFile :: forall a_XrHB.
                      Show a_XrHB =>
                      a_XrHB -> FileSystemAction String,
        in an equation for ‘==’
        at /Users/jeremy/repos/haskell/monad-mock/test-suite/Control/Monad/MockSpec.hs:20:1
      ‘a’ is a rigid type variable bound by
        a pattern with constructor:
          ReadFile :: forall a_XrHB.
                      Show a_XrHB =>
                      a_XrHB -> FileSystemAction String,
        in an equation for ‘==’
        at /Users/jeremy/repos/haskell/monad-mock/test-suite/Control/Monad/MockSpec.hs:20:1
    • In the second argument of ‘(==)’, namely ‘b1’
      In the expression: ((a1 == b1))
      In an equation for ‘==’:
          (==) (ReadFile a1) (ReadFile b1) = ((a1 == b1))
      When typechecking the code for ‘==’
        in a derived instance for ‘Eq (FileSystemAction r)’:
        To see the code I am typechecking, use -ddump-deriv
    • Relevant bindings include
        b1 :: a1
          (bound at /Users/jeremy/repos/haskell/monad-mock/test-suite/Control/Monad/MockSpec.hs:20:1)
        a1 :: a
          (bound at /Users/jeremy/repos/haskell/monad-mock/test-suite/Control/Monad/MockSpec.hs:20:1)

/Users/jeremy/repos/haskell/monad-mock/test-suite/Control/Monad/MockSpec.hs:20:1: warning: [-Wdeferred-type-errors]
    • Could not deduce: a2 ~ a1
      from the context: (a ~ [Char], Show a1)
        bound by a pattern with constructor:
                   ReadFile :: forall a_XrHB.
                               Show a_XrHB =>
                               a_XrHB -> FileSystemAction String,
                 in an equation for ‘eqAction’
        at /Users/jeremy/repos/haskell/monad-mock/test-suite/Control/Monad/MockSpec.hs:20:1-60
      or from: (b ~ [Char], Show a2)
        bound by a pattern with constructor:
                   ReadFile :: forall a_XrHB.
                               Show a_XrHB =>
                               a_XrHB -> FileSystemAction String,
                 in an equation for ‘eqAction’
        at /Users/jeremy/repos/haskell/monad-mock/test-suite/Control/Monad/MockSpec.hs:20:1-60
      ‘a2’ is a rigid type variable bound by
        a pattern with constructor:
          ReadFile :: forall a_XrHB.
                      Show a_XrHB =>
                      a_XrHB -> FileSystemAction String,
        in an equation for ‘eqAction’
        at /Users/jeremy/repos/haskell/monad-mock/test-suite/Control/Monad/MockSpec.hs:20:1
      ‘a1’ is a rigid type variable bound by
        a pattern with constructor:
          ReadFile :: forall a_XrHB.
                      Show a_XrHB =>
                      a_XrHB -> FileSystemAction String,
        in an equation for ‘eqAction’
        at /Users/jeremy/repos/haskell/monad-mock/test-suite/Control/Monad/MockSpec.hs:20:1
    • In the second argument of ‘(==)’, namely ‘y_arVc’
      In the first argument of ‘(&&)’, namely ‘(==) x_arVb y_arVc’
      In the expression: (&&) ((==) x_arVb y_arVc) True
    • Relevant bindings include
        y_arVc :: a2
          (bound at /Users/jeremy/repos/haskell/monad-mock/test-suite/Control/Monad/MockSpec.hs:20:1)
        x_arVb :: a1
          (bound at /Users/jeremy/repos/haskell/monad-mock/test-suite/Control/Monad/MockSpec.hs:20:1)