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
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
|
{- |
Module : Control.Monad.Error.Class
Copyright : (c) Michael Weber <michael.weber@post.rwth-aachen.de> 2001,
(c) Jeff Newbern 2003-2006,
(c) Andriy Palamarchuk 2006
License : BSD-style (see the file libraries/base/LICENSE)
Maintainer : libraries@haskell.org
Stability : experimental
Portability : non-portable (type families)
[Computation type:] Computations which may fail or throw exceptions.
[Binding strategy:] Failure records information about the cause\/location
of the failure. Failure values bypass the bound function,
other values are used as inputs to the bound function.
[Useful for:] Building computations from sequences of functions that may fail
or using exception handling to structure error handling.
[Zero and plus:] Zero is represented by an empty error and the plus operation
executes its second argument if the first fails.
[Example type:] @'Either' 'String' a@
The Error monad (also called the Exception monad).
-}
{-
Rendered by Michael Weber <mailto:michael.weber@post.rwth-aachen.de>,
inspired by the Haskell Monad Template Library from
Andy Gill (<http://web.cecs.pdx.edu/~andy/>)
-}
module Control.Monad.Error.Class (
Error(..),
MonadError(..),
) where
import Control.Monad.Trans.Error (Error(..), ErrorT)
import qualified Control.Monad.Trans.Error as ErrorT (throwError, catchError)
import Control.Monad.Trans.Identity as Identity
import Control.Monad.Trans.List as List
import Control.Monad.Trans.Maybe as Maybe
import Control.Monad.Trans.Reader as Reader
import Control.Monad.Trans.RWS.Lazy as LazyRWS
import Control.Monad.Trans.RWS.Strict as StrictRWS
import Control.Monad.Trans.State.Lazy as LazyState
import Control.Monad.Trans.State.Strict as StrictState
import Control.Monad.Trans.Writer.Lazy as LazyWriter
import Control.Monad.Trans.Writer.Strict as StrictWriter
import Control.Monad.Trans
import Control.Monad
import Control.Monad.Instances ()
import Data.Monoid
import System.IO
{- |
The strategy of combining computations that can throw exceptions
by bypassing bound functions
from the point an exception is thrown to the point that it is handled.
Is parameterized over the type of error information and
the monad type constructor.
It is common to use @'Data.Either' String@ as the monad type constructor
for an error monad in which error descriptions take the form of strings.
In that case and many other common cases the resulting monad is already defined
as an instance of the 'MonadError' class.
You can also define your own error type and\/or use a monad type constructor
other than @'Data.Either' String@ or @'Data.Either' IOError@.
In these cases you will have to explicitly define instances of the 'Error'
and\/or 'MonadError' classes.
-}
class (Monad m) => MonadError m where
type ErrorType m
-- | Is used within a monadic computation to begin exception processing.
throwError :: ErrorType m -> m a
{- |
A handler function to handle previous errors and return to normal execution.
A common idiom is:
> do { action1; action2; action3 } `catchError` handler
where the @action@ functions can call 'throwError'.
Note that @handler@ and the do-block must have the same return type.
-}
catchError :: m a -> (ErrorType m -> m a) -> m a
instance MonadError IO where
type ErrorType IO = IOError
throwError = ioError
catchError = catch
-- ---------------------------------------------------------------------------
-- Our parameterizable error monad
instance (Error e) => MonadError (Either e) where
type ErrorType (Either e) = e
throwError = Left
Left l `catchError` h = h l
Right r `catchError` _ = Right r
instance (Monad m, Error e) => MonadError (ErrorT e m) where
type ErrorType (ErrorT e m) = e
throwError = ErrorT.throwError
catchError = ErrorT.catchError
-- ---------------------------------------------------------------------------
-- Instances for other mtl transformers
instance (MonadError m) => MonadError (IdentityT m) where
type ErrorType (IdentityT m) = ErrorType m
throwError = lift . throwError
catchError = Identity.liftCatch catchError
instance (MonadError m) => MonadError (ListT m) where
type ErrorType (ListT m) = ErrorType m
throwError = lift . throwError
catchError = List.liftCatch catchError
instance (MonadError m) => MonadError (MaybeT m) where
type ErrorType (MaybeT m) = ErrorType m
throwError = lift . throwError
catchError = Maybe.liftCatch catchError
instance (MonadError m) => MonadError (ReaderT r m) where
type ErrorType (ReaderT r m) = ErrorType m
throwError = lift . throwError
catchError = Reader.liftCatch catchError
instance (Monoid w, MonadError m) => MonadError (LazyRWS.RWST r w s m) where
type ErrorType (LazyRWS.RWST r w s m) = ErrorType m
throwError = lift . throwError
catchError = LazyRWS.liftCatch catchError
instance (Monoid w, MonadError m) => MonadError (StrictRWS.RWST r w s m) where
type ErrorType (StrictRWS.RWST r w s m) = ErrorType m
throwError = lift . throwError
catchError = StrictRWS.liftCatch catchError
instance (MonadError m) => MonadError (LazyState.StateT s m) where
type ErrorType (LazyState.StateT s m) = ErrorType m
throwError = lift . throwError
catchError = LazyState.liftCatch catchError
instance (MonadError m) => MonadError (StrictState.StateT s m) where
type ErrorType (StrictState.StateT s m) = ErrorType m
throwError = lift . throwError
catchError = StrictState.liftCatch catchError
instance (Monoid w, MonadError m) => MonadError (LazyWriter.WriterT w m) where
type ErrorType (LazyWriter.WriterT w m) = ErrorType m
throwError = lift . throwError
catchError = LazyWriter.liftCatch catchError
instance (Monoid w, MonadError m) => MonadError (StrictWriter.WriterT w m) where
type ErrorType (StrictWriter.WriterT w m) = ErrorType m
throwError = lift . throwError
catchError = StrictWriter.liftCatch catchError
|
