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 function defintions for performing the operations of T via send. For example,

makeEffect ''Filesystem

generates the following definitions:

readFile      :: Filesystem :> es => FilePath -> Eff es String
readFile  x   =  send (ReadFile x)
writeFile     :: Filesystem :> es => FilePath -> String -> Eff es ()
writeFile x y =  send (WriteFile x y)

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.

Technical details

This function is also "weaker" than polysemy's makeSem, because this function cannot properly handle some cases involving ambiguous types. Those cases are rare, though. See the ThSpec test spec 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.