Control.THEff.Catch

Overview

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

import Control.THEff
import Control.THEff.Catch

mkEff "MyCatch"     ''Catch     ''Float       ''NoEff

foo:: Float -> Eff (MyCatch m String) String
foo x = do
    throwCtchIf  x (==0)
    return $ "1/" ++ show x ++ " = " ++ (show $ 1 / x)
    
hndlr :: Float -> String
hndlr x = "Error : x=" ++ show x

>>> runMyCatch hndlr $ foo 4
"1/4.0 = 0.25"

>>> runMyCatch hndlr $ foo 0
"Error : x=0.0" 

Types and functions used in mkEff

data Catch' v e

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

data Catch 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

CatchOuter (o m e)

CatchAction (Catch' v e)

CatchResult a

type CatchArgT v r = v -> r

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

type CatchResT r = r

Result type of runEEEE.

effCatch :: EffClass Catch' v e => Catch' 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.

runEffCatch

Arguments

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

=> (u t r -> (r -> m (CatchResT z)) -> m (CatchResT z))

The outer effect function

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

The chain of effects link wrapper.

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

The chain of effects link unwrapper.

-> CatchArgT v z

The argument of effect. Checking and/or correction function.

-> Eff r a

-> m (CatchResT z)

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

Functions that use this effect

throwCtch :: EffClass Catch' v e => v -> Eff e ()

Throw effect specific exception.

throwCtchIf :: EffClass Catch' v e => v -> (v -> Bool) -> Eff e ()

Throw effect specific exception if first argument is True.