Control.THEff.Exception

Overview

{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE TemplateHaskell #-}

import Control.THEff
import Control.THEff.Exception
 
mkEff "MyException" ''Except    ''String   ''NoEff

safeSqrt :: Float -> Eff (MyException m Float) Float
safeSqrt x =  do
    throwIf (x<0) "The argument must be non-negative."
    return $ sqrt x 

>>> runMyException id $ safeSqrt 4
Right 2.0

>>> runMyException id $ safeSqrt (-1)
Left "The argument must be non-negative." 

Types and functions used in mkEff

data Except' v e

Actually, the effect type - v - Type - the parameter of the effect. - e - mkEff generated type.

data Except m e o v a

Type implements link in the chain of effects. Constructors must be named {EffectName}{Outer|WriterAction|WriterResult} and have a specified types of fields. - m - Or Monad (if use the Lift) or phantom type - stub (if used NoEff). - o - Type of outer effect. - a - The result of mkEff generated runEEEE... function.

Constructors

ExceptOuter (o m e)

ExceptAction (Except' v e)

ExceptResult a

type ExceptArgT v = v -> v

Type of fourth argument of runEffExcept and first argument of runEEEE.

type ExceptResT r v = Either v r

Result type of runEEEE.

effExcept :: EffClass Except' v e => Except' v r -> Eff e r

This function is used in the mkEff generated runEEEE functions and typically in effect action functions. Calling the effect action.

runEffExcept

Arguments

:: forall (t :: * -> *) (u :: (* -> *) -> * -> *) (m :: * -> *) (m1 :: * -> *) (o :: (* -> *) -> * -> *). Monad m

=> (u t r -> (r -> m (ExceptResT z v)) -> m (ExceptResT z v))

The outer effect function

-> (Except m1 e o w a -> r)

The chain of effects link wrapper.

-> (r -> Except t r u v z)

The chain of effects link unwrapper.

-> ExceptArgT v

The argument of effect. Except value map. Usualy id.

-> Eff r a

-> m (ExceptResT z v)

The main function of the effect implementing. This function is used in the mkEff generated runEEEE functions.

Functions that use this effect

throwExc :: EffClass Except' v e => v -> Eff e ()

Throw effect specific exception.

throwIf :: EffClass Except' v e => Bool -> v -> Eff e ()

Throw effect specific exception if first argument is True.