Instance Effects Trouble

InstanceEffTrouble.purs

module Test.Spec where

import Debug.Trace
import Control.Monad
import Control.Monad.Error.Class
import Control.Monad.Trans
import Control.Monad.Eff
import Control.Monad.Eff.Exception
import Control.Monad.Eff.Class
import Control.Monad.Aff

class TestBody e body where
  toAff :: body -> Aff e Unit

instance testBodyBoolean :: TestBody e Boolean where
  toAff true = pure unit
  toAff false = throwError $ error "Result was false"

instance testBodyAff :: TestBody e (Aff e Unit) where
  toAff a = a

risky :: forall e. Aff e Unit
risky = throwError $ error "OMG NO"

test :: forall e b.
     (TestBody e b) =>
     b -> Aff e Unit
test body = toAff body

trueTest  :: forall e. Aff e Unit
trueTest  = test true

riskyTest :: forall e. Aff e Unit
riskyTest = test risky

run :: forall e. Eff (trace :: Trace | e) Unit
run = do
  runAff print print trueTest
  runAff print print riskyTest

output.txt

Compiling Test.Spec
Error in module Test.Spec:
Error in value declaration riskyTest:
Error at src/Test/Spec.purs line 3, column 1 - line 4, column 1:
  No instance found for Test.Spec.TestBody e57 (Control.Monad.Aff.Aff u60 Prelude.Unit)
See https://github.com/purescript/purescript/wiki/Error-Code-NoInstanceFound for more information, or to contribute content related to this error.