Dynamic values with support for rank-1 polymorphic types.

Examples of fromDynamic

These examples correspond to the isInstanceOf examples in Data.Rank1Typeable.

> do f <- fromDynamic (toDynamic (even :: Int -> Bool)) ; return $ (f :: Int -> Int) 0
Left "Cannot unify Int and Bool"

> do f <- fromDynamic (toDynamic (const 1 :: ANY -> Int)) ; return $ (f :: Int -> Int) 0
Right 1

> do f <- fromDynamic (toDynamic (unsafeCoerce :: ANY1 -> ANY2)) ; return $ (f :: Int -> Int) 0
Right 0

> do f <- fromDynamic (toDynamic (id :: ANY -> ANY)) ; return $ (f :: Int -> Bool) 0
Left "Cannot unify Bool and Int"

> do f <- fromDynamic (toDynamic (undefined :: ANY)) ; return $ (f :: Int -> Int) 0
Right *** Exception: Prelude.undefined

> do f <- fromDynamic (toDynamic (id :: ANY -> ANY)) ; return $ (f :: Int)
Left "Cannot unify Int and ->"

Examples of dynApply

These examples correspond to the funResultTy examples in Data.Rank1Typeable.

> do app <- toDynamic (id :: ANY -> ANY) `dynApply` toDynamic True ; f <- fromDynamic app ; return $ (f :: Bool)
Right True

> do app <- toDynamic (const :: ANY -> ANY1 -> ANY) `dynApply` toDynamic True ; f <- fromDynamic app ; return $ (f :: Int -> Bool) 0
Right True

> do app <- toDynamic (($ True) :: (Bool -> ANY) -> ANY) `dynApply` toDynamic (id :: ANY -> ANY) ; f <- fromDynamic app ; return (f :: Bool)
Right True

> app <- toDynamic (const :: ANY -> ANY1 -> ANY) `dynApply` toDynamic (id :: ANY -> ANY) ; f <- fromDynamic app ; return $ (f :: Int -> Bool -> Bool) 0 True
Right True

> do app <- toDynamic ((\f -> f . f) :: (ANY -> ANY) -> ANY -> ANY) `dynApply` toDynamic (even :: Int -> Bool) ; f <- fromDynamic app ; return (f :: ())
Left "Cannot unify Int and Bool"

Using toDynamic

When using polymorphic values you need to give an explicit type annotation:

> toDynamic id

<interactive>:46:1:
    Ambiguous type variable `a0' in the constraint:
      (Typeable a0) arising from a use of `toDynamic'
    Probable fix: add a type signature that fixes these type variable(s)
    In the expression: toDynamic id
    In an equation for `it': it = toDynamic id

versus

> toDynamic (id :: ANY -> ANY)
<<ANY -> ANY>>

Note that these type annotation are checked by ghc like any other:

> toDynamic (id :: ANY -> ANY1)

<interactive>:45:12:
    Couldn't match expected type `V1' with actual type `V0'
    Expected type: ANY -> ANY1
      Actual type: ANY -> ANY
    In the first argument of `toDynamic', namely `(id :: ANY -> ANY1)'
    In the expression: toDynamic (id :: ANY -> ANY1)