Cleff.Internal.TH

This module contains Template Haskell functions for generating definitions of functions that send effect operations. You mostly won't want to import this module directly; The Cleff module reexports the main functionalities of this module.

This is an internal module and its API may change even between minor versions. Therefore you should be extra careful if you're to depend on this module.

Documentation

makeEffect :: Name -> Q [Dec]

For a datatype T representing an effect, makeEffect T generates functions defintions for performing the operations of T via send. The naming rule is changing the first uppercase letter in the constructor name to lowercase or removing the : symbol in the case of operator constructors. Also, this function will preserve any fixity declarations defined on the constructors.

Because of the limitations of Template Haskell, all constructors of T should be polymorphic in the monad type, if they are to be used by makeEffect. For example, this is not OK:

data Limited :: Effect where
  Noop :: Limited (Eff es) ()

because the monad type Eff es is not a fully polymorphic type variable.

This function is also "weaker" than polysemy's makeSem, because this function cannot properly handle some cases involving complex higher order effects. Those cases are rare, though. See the tests for more details.

makeEffect_ :: Name -> Q [Dec]

Like makeEffect, but doesn't generate type signatures. This is useful when you want to attach Haddock documentation to the function signature, e.g.:

data Identity :: Effect where
  Noop :: Identity m ()
makeEffect_ ''Identity

-- | Perform nothing at all.
noop :: Identity :> es => Eff es ()

Be careful that the function signatures must be added after the makeEffect_ call.

makeSmartCons :: Bool -> Name -> Q [Dec]

This is the function underlying makeEffect and makeEffect_. You can switch between the behavior of two by changing the Bool parameter to True (generating signatures) or False (not generating signatures).

makeCon :: Bool -> Name -> Q [Dec]

Make a single function definition of a certain effect operation.