| Safe Haskell | None |
|---|---|
| Language | Haskell2010 |
Control.Monad.Log
Contents
- logMessage :: MonadLog message m => message -> m ()
- mapLogMessage :: MonadLog message' m => (message -> message') -> LoggingT message m a -> m a
- mapLogMessageM :: MonadLog message' m => (message -> m message') -> LoggingT message m a -> m a
- class Monad m => MonadLog message m | m -> message where
- logDebug :: MonadLog (WithSeverity a) m => a -> m ()
- logInfo :: MonadLog (WithSeverity a) m => a -> m ()
- logNotice :: MonadLog (WithSeverity a) m => a -> m ()
- logWarning :: MonadLog (WithSeverity a) m => a -> m ()
- logError :: MonadLog (WithSeverity a) m => a -> m ()
- logCritical :: MonadLog (WithSeverity a) m => a -> m ()
- logAlert :: MonadLog (WithSeverity a) m => a -> m ()
- logEmergency :: MonadLog (WithSeverity a) m => a -> m ()
- renderPretty :: Float -> Int -> Doc -> SimpleDoc
- data WithTimestamp a = WithTimestamp {
- discardTimestamp :: a
- msgTimestamp :: UTCTime
- timestamp :: MonadIO m => a -> m (WithTimestamp a)
- renderWithTimestamp :: (UTCTime -> String) -> (a -> Doc) -> WithTimestamp a -> Doc
- data WithSeverity a = WithSeverity {
- msgSeverity :: Severity
- discardSeverity :: a
- data Severity
- renderWithSeverity :: (a -> Doc) -> WithSeverity a -> Doc
- data WithCallStack a = WithCallStack {}
- withCallStack :: (?stack :: CallStack) => a -> WithCallStack a
- renderWithCallStack :: (a -> Doc) -> WithCallStack a -> Doc
- newtype LoggingT message m a = LoggingT (ReaderT (Handler m message) m a)
- runLoggingT :: LoggingT message m a -> Handler m message -> m a
- mapLoggingT :: (forall x. (Handler m message -> m x) -> Handler n message' -> n x) -> LoggingT message m a -> LoggingT message' n a
- type Handler m message = message -> m ()
- withFDHandler :: (MonadIO io, MonadMask io) => BatchingOptions -> Handle -> Float -> Int -> (Handler io Doc -> io a) -> io a
- withBatchedHandler :: (MonadIO io, MonadMask io) => BatchingOptions -> (NonEmpty message -> IO ()) -> (Handler io message -> io a) -> io a
- data BatchingOptions = BatchingOptions {}
- defaultBatchingOptions :: BatchingOptions
- newtype PureLoggingT log m a = MkPureLoggingT (StateT log m a)
- runPureLoggingT :: Monoid log => PureLoggingT log m a -> m (a, log)
- newtype DiscardLoggingT message m a = DiscardLoggingT {
- discardLogging :: m a
Introduction
logging-effect provides a toolkit for general logging in Haskell programs
and libraries. The library consists of the type class MonadLog to add log
output to computations, and this library comes with a set of instances to help
you decide how this logging should be performed. There are predefined handlers
to write to file handles, to accumulate logs purely, or to discard logging
entirely.
Unlike other logging libraries available on Hackage, MonadLog does not
assume that you will be logging text information. Instead, the choice of logging
data is up to you. This leads to a highly compositional form of logging, with
the able to reinterpret logs into different formats, and avoid throwing
information away if your final output is structured (such as logging to a
relational database).
Getting Started
logging-effect is designed to be used via the MonadLog type class and
encourages an "mtl" style approach to programming. If you're not familiar with
the mtl, this approach uses type classes to keep the choice of monad
polymorphic as you program, and you later choose a specific monad transformer
stack when you execute your program. For more information, see
Aside: An mtl refresher.
Working with logging-effect
Emitting log messages
To add logging to your applications, you will need to make two changes.
First, use the MonadLog type class to indicate that a computation has
access to logging. MonadLog is parameterized on the type of messages
that you intend to log. In this example, we will log a Doc that is
wrapped in WithSeverity.
testApp ::MonadLog(WithSeverityDoc) m => m () testApp = do logMessage (WithSeverityInformational"Don't mind me") logMessage (WithSeverityError"But do mind me!")
Note that this does not specify where the logs "go", we'll address that when we run the program.
Outputting with LoggingT
Next, we need to run this computation under a MonadLog effect handler. The
most flexible handler is LoggingT. LoggingT runs a MonadLog computation
by providing it with a Handler, which is a computation that can be in the
underlying monad.
For example, we can easily fulfill the MonadLog type class by just using
print as our Handler:
>>>runLoggingT testApp printWithSeverity {msgSeverity = Informational, discardSeverity = "Don't mind me"} WithSeverity {msgSeverity = Error, discardSeverity = "But do mind me!"}
The log messages are printed according to their Show instances, and - while
this works - it is not particularly user friendly. As Handlers are just functions
from log messages to monadic actions, we can easily reformat log messages.
logging-effect comes with a few "log message transformers" (such as
WithSeverity), and each of these message transformers has a canonical way to
render in a human-readable format:
>>>runLoggingT testApp (print . renderWithSeverity id)[Informational] Don't mind me [Error] But do mind me!
That's looking much more usable - and in fact this approach is probably fine for command line applications.
However, for longer running high performance applications there is a slight
problem. Remember that runLoggingT simply interleaves the given Handler
whenever logMessage is called. By providing print as a Handler, our
application will actually block until the log is complete. This is undesirable
for high performance applications, where it's much better to log asynchronously.
logging-effect comes with "batched handlers" for this problem. Batched handlers
are handlers that log asynchronously, are flushed periodically, and have maximum
memory impact. Batched handlers are created with withBatchedHandler, though
if you are just logging to file descriptors you can also use withFDHandler.
We'll use this next to log to STDOUT:
main ::IO() main =withFDHandlerdefaultBatchingOptionsstdout0.4 80 $ \logToStdout ->runLoggingTtestApp (logToStdout.renderWithSeverityid)
Finally, as Handlers are just functions (we can't stress this enough!) you
are free to slice-and-dice your log messages however you want. As our log
messages are structured, we can pattern match on the messages and dispatch them
to multiple handlers. In this final example of using LoggingT we'll split
our log messages between STDOUT and STDERR, and change the formatting of
error messages:
main :: IO () main = dowithFDHandlerdefaultBatchingOptionsstderr0.4 80 $ \stderrHandler ->withFDHandlerdefaultBatchingOptionsstdout0.4 80 $ \stdoutHandler ->runLoggingTtestApp (\message -> casemsgSeveritymessage ofError-> stderrHandler (discardSeveritymessage) _ -> stdoutHandler (renderWithSeverityid message))
>>>main[Informational] Don't mind me! BUT DO MIND ME!
Adapting and composing logging
So far we've considered very small applications where all log messages fit nicely
into a single type. However, as applications grow and begin to reuse components,
it's unlikely that this approach will scale. logging-effect comes with a
mapping function - mapLogMessage - which allows us to map log messages from one
type to another (just like how we can use map to change elements of a list).
For example, we've already seen the basic testApp computation above that used
WithSeverity to add severity information to log messages. Elsewhere we might
have some older code that doesn't yet have any severity information:
legacyCode ::MonadLogDocm => m () legacyCode =logMessage"Does anyone even remember writing this function?"
Here legacyCode is only logging Doc, while our testApp is logging
WithSeverity Doc. What happens if we compose these programs?
>>>:t testApp >> legacyCodeCouldn't match type ‘Doc’ with ‘WithSeverity Doc’
Whoops! MonadLog has functional dependencies on the type class which means
that there can only be a single way to log per monad. One solution might be
to lift one set of logs into the other:
>>>:t testApp >> lift legacyCode:: (MonadTrans t, MonadLog Doc m, MonadLog (WithSeverity Doc) (t m)) => t m ()
And indeed, this is a solution, but it's not a particularly nice one.
Instead, we can map both of these computations into a common log format:
>>>:t mapLogMessage Left testApp >> mapLogMessage Right (logMessage "Hello"):: (MonadLog (Either (WithSeverity Doc) Doc) m) => m ()
This is a trivial way of combining two different types of log message. In larger applications you will probably want to define a new sum-type that combines all of your log messages, and generally sticking with a single log message type per application.
MonadLog
logMessage :: MonadLog message m => message -> m () Source #
Append a message to the log for this computation.
mapLogMessage :: MonadLog message' m => (message -> message') -> LoggingT message m a -> m a Source #
Re-interpret the log messages in one computation. This can be useful to embed a computation with one log type in a larger general computation.
mapLogMessageM :: MonadLog message' m => (message -> m message') -> LoggingT message m a -> m a Source #
Monadic version of mapLogMessage. This can be used to annotate a
message with something that can only be computed in a monad. See e.g.
timestamp.
class Monad m => MonadLog message m | m -> message where Source #
The class of monads that support logging.
Laws:
Monoid homomorphism:
logMessageFreea*>logMessageFreeb =logMessageFree(a<>b)
Methods
logMessageFree :: (forall n. Monoid n => (message -> n) -> n) -> m () Source #
Fold log messages into this computation. Looking to just log a
message? You probably want logMessage.
The perhaps strange type here allows us to construct a monoid out of any type of log message. You can think of this as the simpler type:
logMessageFree :: [message] -> m ()
logMessageFree :: (m ~ t n, MonadTrans t, MonadLog message n) => (forall mon. Monoid mon => (message -> mon) -> mon) -> m () Source #
Fold log messages into this computation. Looking to just log a
message? You probably want logMessage.
The perhaps strange type here allows us to construct a monoid out of any type of log message. You can think of this as the simpler type:
logMessageFree :: [message] -> m ()
Instances
| MonadLog message m => MonadLog message (CatchT m) Source # | |
| MonadLog message m => MonadLog message (ListT m) Source # | |
| MonadLog message m => MonadLog message (MaybeT m) Source # | |
| Monad m => MonadLog message (DiscardLoggingT message m) Source # | The trivial instance of |
| (Monad m, Monoid log) => MonadLog log (PureLoggingT log m) Source # | A pure handler of |
| Monad m => MonadLog message (LoggingT message m) Source # | The main instance of |
| (Functor f, MonadLog message m) => MonadLog message (FT f m) Source # | |
| (Functor f, MonadLog message m) => MonadLog message (FreeT f m) Source # | |
| (Error e, MonadLog message m) => MonadLog message (ErrorT e m) Source # | |
| MonadLog message m => MonadLog message (ExceptT e m) Source # | |
| (Monoid w, MonadLog message m) => MonadLog message (WriterT w m) Source # | |
| (Monoid w, MonadLog message m) => MonadLog message (WriterT w m) Source # | |
| MonadLog message m => MonadLog message (StateT s m) Source # | |
| MonadLog message m => MonadLog message (StateT s m) Source # | |
| MonadLog message m => MonadLog message (IdentityT * m) Source # | |
| MonadLog message m => MonadLog message (ContT * r m) Source # | |
| MonadLog message m => MonadLog message (ReaderT * r m) Source # | |
| (Monoid w, MonadLog message m) => MonadLog message (RWST r w s m) Source # | |
| (Monoid w, MonadLog message m) => MonadLog message (RWST r w s m) Source # | |
Convenience logging combinators
While logging-effect tries to be as general as possible, there is a fairly
common case of logging, namely basic messages with an indication of severity.
These combinators assume that you will be using WithSeverity at the outer-most
level of your log message stack, though no make no assumptions at what is inside
your log messages. There is a logX combinator for each level in Severity.
logDebug :: MonadLog (WithSeverity a) m => a -> m () Source #
logInfo :: MonadLog (WithSeverity a) m => a -> m () Source #
logNotice :: MonadLog (WithSeverity a) m => a -> m () Source #
logWarning :: MonadLog (WithSeverity a) m => a -> m () Source #
logError :: MonadLog (WithSeverity a) m => a -> m () Source #
logCritical :: MonadLog (WithSeverity a) m => a -> m () Source #
logAlert :: MonadLog (WithSeverity a) m => a -> m () Source #
logEmergency :: MonadLog (WithSeverity a) m => a -> m () Source #
Message transformers
renderPretty :: Float -> Int -> Doc -> SimpleDoc #
This is the default pretty printer which is used by show,
putDoc and hPutDoc. (renderPretty ribbonfrac width x)
renders document x with a page width of width and a ribbon
width of (ribbonfrac * width) characters. The ribbon width is
the maximal amount of non-indentation characters on a line. The
parameter ribbonfrac should be between 0.0 and 1.0. If it
is lower or higher, the ribbon width will be 0 or width
respectively.
Timestamps
data WithTimestamp a Source #
Add a timestamp to log messages.
Note that while most log message transformers are designed to be used at the
point of logging, this transformer is best applied within the handler.
This is advised as timestamps are generally applied uniformly, so doing it
in the handler is fine (no extra information or context of the program is
required). The other reason is that logging with a timestamp requires
MonadIO - while the rest of your computation is free to use MonadIO,
it's best to avoid incurring this constraint as much as possible, as it is
generally untestable.
Constructors
| WithTimestamp | |
Fields
| |
Instances
| Functor WithTimestamp Source # | |
| Foldable WithTimestamp Source # | |
| Traversable WithTimestamp Source # | |
| Eq a => Eq (WithTimestamp a) Source # | |
| Ord a => Ord (WithTimestamp a) Source # | |
| Read a => Read (WithTimestamp a) Source # | |
| Show a => Show (WithTimestamp a) Source # | |
timestamp :: MonadIO m => a -> m (WithTimestamp a) Source #
Add the current time as a timestamp to a message.
Arguments
| :: (UTCTime -> String) | How to format the timestamp. |
| -> (a -> Doc) | How to render the rest of the message. |
| -> WithTimestamp a -> Doc |
Given a way to render the underlying message a and a way to format
UTCTime, render a message with its timestamp.
>>>renderWithTimestamp (formatTime defaultTimeLocale rfc822DateFormat) id timestamppedLogMessage[Tue, 19 Jan 2016 11:29:42 UTC] Setting target speed to plaid
Severity
data WithSeverity a Source #
Add "Severity" information to a log message. This is often used to convey how significant a log message is.
Constructors
| WithSeverity | |
Fields
| |
Instances
| Functor WithSeverity Source # | |
| Foldable WithSeverity Source # | |
| Traversable WithSeverity Source # | |
| Eq a => Eq (WithSeverity a) Source # | |
| Ord a => Ord (WithSeverity a) Source # | |
| Read a => Read (WithSeverity a) Source # | |
| Show a => Show (WithSeverity a) Source # | |
Classes of severity for log messages. These have been chosen to match
syslog severity levels
Constructors
| Emergency | System is unusable. By |
| Alert | Should be corrected immediately. |
| Critical | Critical conditions. |
| Error | Error conditions. |
| Warning | May indicate that an error will occur if action is not taken. |
| Notice | Events that are unusual, but not error conditions. |
| Informational | Normal operational messages that require no action. |
| Debug | Information useful to developers for debugging the application. |
renderWithSeverity :: (a -> Doc) -> WithSeverity a -> Doc Source #
Given a way to render the underlying message a, render a message with its
severity.
>>>renderWithSeverity id (WithSeverity Informational "Flux capacitor is functional")[Informational] Flux capacitor is functional
Call stacks
data WithCallStack a Source #
Add call stack information to log lines.
This functional requires that you pass around the call stack via implicit parameters. For more information, see the GHC manual (section 9.14.4.5).
Constructors
| WithCallStack | |
Fields
| |
Instances
withCallStack :: (?stack :: CallStack) => a -> WithCallStack a Source #
Construct a WithCallStack log message.
This should normally be preferred over just using WithCallStack as it will
append a new entry to the stack - pointing to this exact log line. However,
if you are creating a combinator (such as a wrapper that logs and throws
an exception), you may be better manually capturing the CallStack and
using WithCallStack.
renderWithCallStack :: (a -> Doc) -> WithCallStack a -> Doc Source #
Given a way to render the underlying message a render a message with a
callstack.
The callstack will be pretty-printed underneath the log message itself.
LoggingT, a general handler
newtype LoggingT message m a Source #
LoggingT is a very general handler for the MonadLog effect. Whenever a
log entry is emitted, the given Handler is invoked, producing some
side-effect (such as writing to stdout, or appending a database table).
Instances
runLoggingT :: LoggingT message m a -> Handler m message -> m a Source #
Given a Handler for a given message, interleave this Handler into the
underlying m computation whenever logMessage is called.
mapLoggingT :: (forall x. (Handler m message -> m x) -> Handler n message' -> n x) -> LoggingT message m a -> LoggingT message' n a Source #
LoggingT Handlers
type Handler m message = message -> m () Source #
Handlers are mechanisms to interpret the meaning of logging as an action
in the underlying monad. They are simply functions from log messages to
m-actions.
Arguments
| :: (MonadIO io, MonadMask io) | |
| => BatchingOptions | |
| -> Handle | The |
| -> Float | The |
| -> Int | The amount of characters per line. Lines longer than this will be pretty-printed across multiple lines if possible. |
| -> (Handler io Doc -> io a) | |
| -> io a |
withFDHandler creates a new Handler that will append a given file
descriptor (or Handle, as it is known in the "base" library). Note that
this Handler requires log messages to be of type Doc. This abstractly
specifies a pretty-printing for log lines. The two arguments two
withFDHandler determine how this pretty-printing should be realised
when outputting log lines.
These Handlers asynchronously log messages to the given file descriptor,
rather than blocking.
Batched handlers
withBatchedHandler :: (MonadIO io, MonadMask io) => BatchingOptions -> (NonEmpty message -> IO ()) -> (Handler io message -> io a) -> io a Source #
Create a new batched handler. Batched handlers take batches of messages to log at once, which can be more performant than logging each individual message.
A batched handler flushes under three criteria:
- The flush interval has elapsed and the queue is not empty.
- The queue has become full and needs to be flushed.
- The scope of
withBatchedHandleris exited.
Batched handlers queue size and flush period can be configured via
BatchingOptions.
data BatchingOptions Source #
Options that be used to configure withBatchingHandler.
Constructors
| BatchingOptions | |
Fields
| |
defaultBatchingOptions :: BatchingOptions Source #
Defaults for BatchingOptions
defaultBatchingOptions=BatchingOptions{flushMaxDelay= 1000000 ,flushMaxQueueSize= 100 ,blockWhenFull=True}
Pure logging
newtype PureLoggingT log m a Source #
A MonadLog handler optimised for pure usage. Log messages are accumulated
strictly, given that messasges form a Monoid.
Constructors
| MkPureLoggingT (StateT log m a) |
Instances
runPureLoggingT :: Monoid log => PureLoggingT log m a -> m (a, log) Source #
Run a computation with access to logging by accumulating a log under its
Monoid instance.
Discarding logs
newtype DiscardLoggingT message m a Source #
A MonadLog handler that throws messages away.
The documentation may appear a bit confusing, but note that the full type of
discardLogging is:
discardLogging::DiscardLoggingTmesssage m a -> m a
Constructors
| DiscardLoggingT | |
Fields
| |
Instances
Aside: An mtl refresher
If you are already familiar with the mtl you can skip this section. This is not
designed to be an exhaustive introduction to the mtl library, but hopefully
via a short example you'll have a basic familarity with the approach.
In this example, we'll write a program with access to state and general IO
actions. One way to do this would be to work with monad transformers, stacking
StateT on top of IO:
import Control.Monad.Trans.State.Strict (StateT,get,put) import Control.Monad.Trans.Class (lift) transformersProgram ::StateTIntIO() transformersProgram = do stateNow <-getliftlaunchMisslesput(stateNow + 42)
This is OK, but it's not very flexible. For example, the transformers library actually provides us with two implementations of state monads - strict and a lazy variant. In the above approach we have forced the user into a choice (we chose the strict variant), but this can be undesirable. We could imagine that in the future there may be even more implementations of state monads (for example, a state monad that persists state entirely on a remote machine) - if requirements change we are unable to reuse this program without changing its type.
With the mtl, we instead program to an abstract specification of the effects
we require, and we postpone the choice of handler until the point when the
computation is ran.
Rewriting the transformersProgram using the mtl, we have the following:
import Control.Monad.State.Class (MonadState(get,put)) import Control.Monad.IO.Class (MonadIO(liftIO)) mtlProgram :: (MonadStateIntm,MonadIOm) => m () mtlProgram = do stateNow <-getliftIOlaunchMisslesput(stateNow + 42)
Notice that mtlProgram doesn't specify a concrete choice of state monad. The
"transformers" library gives us two choices - strict or lazy state monads. We
make the choice of a specific monad stack when we run our program:
import Control.Monad.Trans.State.Strict (execStateT) main ::IO() main =execStateTmtlProgram 99
Here we chose the strict variant via execStateT. Using execStateT
eliminates the MonadState type class from mtlProgram, so now we only have
to fulfill the MonadIO obligation. There is only one way to handle this, and
that's by working in the IO monad. Fortunately we're inside the main
function, which is in the IO monad, so we're all good.