Nrthugu

I am trying to create a function to generate a string representation of effects at runtime.

-- In a shared module

... definitions for TestConfig, RunConfig



data GenericTest tc rc i effs as vs = GenericTest {

  configuration :: tc,

  components :: ItemClass i vs => TestComponents rc i effs as vs

} deriving Typeable



type Test = GenericTest TestConfig RunConfig



type EFFLogger effs = Member Logger effs

type EFFFileSystem effs = Members '[Logger, Ensure, FileSystem] effs 

-- Child module 1

.... definitions for items, iterator etc 



type Effects effs = EFFFileSystem effs



test :: forall effs. Effects effs => Test Item effs ApState ValState

test = GenericTest {

              configuration = config {address = moduleOf ''ApState},

              components = TestComponents {

                                testItems = items,

                                testInteractor = interactor,

                                testPrepState = prepState

                            }

            }

-- Child module 2 (the same as module 1 but different effects)

.... definitions for items, iterator etc 



type Effects effs = EFFLogger effs



test :: forall effs. Effects effs => Test Item effs ApState ValState

test = GenericTest {

              configuration = config {address = moduleOf ''ApState},

              components = TestComponents {

                                testItems = items,

                                testInteractor = interactor,

                                testPrepState = prepState

                            }

            }

At run time I want a function f such that:

> f ChildMod1.test 

> ["Logger", "Ensure", "FileSystem"]

>

> f ChildMod2.test 

>  ["Logger"]

From repl with Child module 1 loaded I can get the following, which if I could get something similar in non-interpreted code would be enough to get me what I want:

> :t test

> test

    :: (Data.OpenUnion.Internal.FindElem Logger effs,

        Data.OpenUnion.Internal.FindElem Ensure effs,

        Data.OpenUnion.Internal.FindElem FileSystem effs) =>

       Test Item effs ApState ValState

I have tried using Typeable as suggested in the following:

How can I read the metadata of a type at runtime?

but typeOf gives me issues which I have no clue how to solve:

> typeOf test



  <interactive>:5:1-11: error:

      * No instance for (Typeable effs0) arising from a use of `typeOf'

      * In the expression: typeOf test

        In an equation for `it': it = typeOf test



  <interactive>:5:8-11: error:

      * Ambiguous type variable `effs0' arising from a use of `test'

        prevents the constraint `(Data.OpenUnion.Internal.FindElem

                                    Logger effs0)' from being solved.

        Probable fix: use a type annotation to specify what `effs0' should be.

        These potential instances exist:

          two instances involving out-of-scope types

            instance [overlappable] Data.OpenUnion.Internal.FindElem t r =>

                                    Data.OpenUnion.Internal.FindElem t (t' : r)

              -- Defined in `Data.OpenUnion.Internal'

            instance Data.OpenUnion.Internal.FindElem t (t : r)

              -- Defined in `Data.OpenUnion.Internal'

      * In the first argument of `typeOf', namely `test'

        In the expression: typeOf test

        In an equation for `it': it = typeOf test

First, given a (type-level) list of effects effs, we can obtain a string of it via Typeable:

import Type.Reflection



showEffs :: forall effs. Typeable effs => String

showEffs = show (typeRep @effs)

Now the problem is for a function f to grab the constraint in the type of its argument. As you have witnessed, a naive attempt will fail: f test will specialize test and propagate the constraints upwards, resulting in errors about instance resolution and ambiguous type variables.

A better solution is to replace => with a regular data type, which is "matchable".

newtype WithEffects_ es0 es1 a = WithEffects { unWithEffects :: Members es0 es1 => a }

The type synonyms also need some refactoring.

type EFileSystem = '[Logger, Ensure, FileSystem]

type WithEffects = WithEffects_ EFileSystem

Now a test looks like this:

test :: forall effs. WithEffects effs (Test Item effs ApState ValState)

test = WithEffects $ ... -- the rest unchanged

and you'll need to unwrap it explicitly with unWithEffects test. We can now extract a runtime representation of the effects es0 from WithEffects_ es0 es1 a:

import Type.Reflection



effsRepTest :: Typeable es0 => WithEffects_ es0 es1 a -> TypeRep es

effsRepTest _ = typeRep



showEffsTest :: Typeable es0 => WithEffects_ es0 es1 a -> String

showEffsTest = show . effsRepTest

So, to extract a string representing es0, we can write:

showEffsTest test :: String

EDITED: you may find traces of an old version of this answer in the comments, which proposed to use newtype c ==> a = Arr { unArr :: c => a }, but that doesn't work here as Members is a type family. So you need another type that carries the list of effects more explicitly es0 like WithEffects_.

EDITED, again:

Here is a minimal compilable gist: https://gist.github.com/Lysxia/d7b6bdc23bcb43cb40439b7e037e8145

The above answer actually prints this:

': (* -> *) Logger (': (* -> *) Ensure (': (* -> *) FileSystem (' (* -> *))))

For a better looking result, I implemented a custom printer in that gist (the ShowTypes class).