How to unify these types?

Question

I have the following code from purescript-express (but the question is more general).

setHandler :: forall e. Handler e
setHandler = do
    idParam   <- getRouteParam "id"
    send "Yeah! "

appSetup :: forall e. App e
appSetup = do
    get "/set/:id" setHandler

setHandler needs to have the given signature as get is defined as

> :t get
forall e r.
(RoutePattern r) => r
                    -> HandlerM ( express :: EXPRESS | e ) Unit
                    -> AppM ( express :: EXPRESS | e ) Unit

Now however I want to use the following function within setHandler

getPointsSet :: forall f. String -> Aff ( fs :: FS | f ) Foobar

which will give me the following compiler error

[1/1 TypesDoNotUnify] src/Main.purs:31:5

          v
  31      send "Yeah! "
          ^

  Could not match type

    HandlerM

  with type

    Aff

  while trying to match type HandlerM
                               ( express :: EXPRESS
                               | _2
                               )
    with type Aff
                ( fs :: FS
                | _0
                )
  while checking that expression send "Yeah! "
    has type Aff
               ( fs :: FS
               | _0
               )
               _1
  in value declaration setHandler

I understand that using getPointsSet effectively requires setHandler to become a Aff as well but than I cannot wire it up with the get then.

Edit

If I try to add a liftAff as suggested in the answer below

setHandler :: forall e. Handler e
setHandler = do
    idParam   <- getRouteParam "id"
    liftAff $ getPointsSet "../some-data.csv"
    send "Yeah! "

I get the following error

[1/1 NoInstanceFound] src/Main.purs:28:1
   28  setHandler :: forall e. Handler e
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   No type class instance was found for
   Control.Monad.Aff.Class.MonadAff ( fs :: FS | _0 )
                                 (HandlerM ( express :: EXPRESS | e0 ))

The instance head contains unknown type variables. Consider adding a type annotation.
in value declaration setHandler

What do I need to do to resolve that?

Максим Зималиев Максим Зималиев · Accepted Answer · 2016-03-21T20:10:05

It seems that HandlerM has instance of MonadAff, so you can use liftAff. Like here:

setHandler :: forall e. Handler e
setHandler = do
  idParam   <- getRouteParam "id"
  liftAff $ getPointsSet "foo"
  send "Yeah! "

Ah, sorry, rows don't unify without additional annotation of appSetup and setHandler

updated version

appSetup :: forall e. App (fs :: FS|e)
appSetup =
  get "/set/:id" setHandler

setHandler :: forall e. Handler (fs :: FS|e)
setHandler = do
  idParam   <- getRouteParam "id"
  liftAff $ getPointsSet "../some-data.csv"
  send "Yeah! "

You need to alter main type also

main :: forall e. Eff (express :: EXPRESS, fs :: FS|e) Unit

How to unify these types?

2 Answers