blob: 872a8aec0c625daf057a429cd410900ee10ed49b (
plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
|
{-# LANGUAGE CPP #-}
{-# LANGUAGE DeriveFunctor #-}
-- BNF Converter: Error Monad
-- Copyright (C) 2004 Author: Aarne Ranta
-- This file comes with NO WARRANTY and may be used FOR ANY PURPOSE.
module GFCC.ErrM where
-- Control.Monad.Fail import will become redundant in GHC 8.8+
import qualified Control.Monad.Fail as Fail
import Control.Monad (ap)
-- the Error monad: like Maybe type with error msgs
data Err a = Ok a | Bad String
deriving (Read, Show, Eq, Functor)
instance Applicative Err where
pure = Ok
(<*>) = ap
instance Monad Err where
return = Ok
Ok a >>= f = f a
Bad s >>= f = Bad s
#if !(MIN_VERSION_base(4,13,0))
fail = Bad
#endif
instance Fail.MonadFail Err where
fail = Bad
|
