core-program- Opinionated Haskell Interoperability
Safe HaskellNone



Output and Logging from your program.

Broadly speaking, there are two kinds of program: console tools invoked for a single purpose, and long-running daemons that effectively run forever.

Tools tend to be run to either have an effect (in which case they tend not to a say much of anything) or to report a result. This tends to be written to "standard output"—traditionally abbreviated in code as stdout—which is usually printed to your terminal.

Daemons, on the other hand, don't write their output to file descriptor 1; rather they tend to respond to requests by writing to files, replying over network sockets, or sending up smoke signals (ECPUTOOHOT, in case you're curious). What daemons do output, however, is log messages.

While there are many sophisticated logging services around that you can interact with directly, from the point of view of an individual program these tend to have faded away and have become more an aspect of the Infrastructure- or Platform-as-a-Service you're running on. Over the past few years containerization mechanisms like docker, then more recently container orchestration layers like kubernetes, have generally simply captured programs' standard output as if it were the program's log output and then sent that down external logging channels to whatever log analysis system is available. Even programs running locally under systemd or similar tend to follow the same pattern; services write to stdout and that output, as "logs", ends up being fed to the system journal.

So with that in mind, in your program you will either be outputting results to stdout or not writing there at all, and you will either be describing extensively what your application is up to, or not at all.

There is also a "standard error" file descriptor available. We recommend not using it. At best it is unclear what is written to stderr and what isn't; at worse it is lost as many environments in the wild discard stderr entirely. To avoid this most of the time people just combine them in the invoking shell with 2>&1, which inevitably results in stderr text appearing in the middle of normal stdout lines corrupting them.

The original idea of standard error was to provde a way to report adverse conditions without interrupting normal text output, but as we have just observed if it happens without context or out of order there isn't much point. Instead this library offers a mechanism which caters for the different kinds of output in a unified, safe manner.

Three kinds of output/logging messages

Standard output

Your program's normal output to the terminal. This library provides the write (and writeS and writeR) functions to send output to stdout.

Informational messages

When running a tool, you sometimes need to know what it is doing as it is carrying out its steps. The info function allows you to emit descriptive messages to the log channel tracing the activities of your program.

Ideally you would never need to turn this on in a command-line tool, but sometimes a user or operations engineer needs to see what an application is up to. These should be human readable status messages to convey a sense of progress.

In the case of long-running daemons, info can be used to describe high-level lifecycle events, to document individual requests, or even describe individual transitions in a request handler's state machine, all depending on the nature of your program.


Programmers, on the other hand, often need to see the internal state of the program when debugging.

You almost always you want to know the value of some variable or parameter, so the debug (and debugS and debugR) utility functions here send log messages to the console prefixed with a label that is, by convention, the name of the value you are examining.

The important distinction here is that such internal values are almost never useful for someone other than the person or team who wrote the code emitting it. Operations engineers might be asked by developers to turn on --debuging and report back the results; but a user of your program is not going to do that in and of themselves to solve a problem.

Single output channel

It is the easy to make the mistake of having multiple subsystems attempting to write to stdout and these outputs corrupting each other, especially in a multithreaded language like Haskell. The output actions described here send all output to terminal down a single thread-safe channel. Output will be written in the order it was executed, and (so long as you don't use the stdout Handle directly yourself) your terminal output will be sound.

Passing --verbose on the command-line of your program will cause info to write its tracing messages to the terminal. This shares the same output channel as the write* functions and will not cause corruption of your program's normal output.

Passing --debug on the command-line of your program will cause the debug* actions to write their debug-level messages to the terminal. This shares the same output channel as above and again will not cause corruption of your program's normal output.


You can change the current logging level from within your program by calling setVerbosityLevel. You can also toggle between normal Output, Verbose output and Debug logging by sending the SIGUSR1 signal to the program using kill:

$ kill -USR1 42069


data Verbosity Source #

The verbosity level of the output logging subsystem. You can override the level specified on the command-line by calling setVerbosityLevel from within the Program monad.



Since: 0.2.12


Since: 0.4.6


Instances details
Show Verbosity Source # 
Instance details

Defined in Core.Program.Context

Normal output

write :: Rope -> Program τ () Source #

Write the supplied text to stdout.

This is for normal program output.

     write "Beginning now"

writeS :: Show α => α -> Program τ () Source #

Call show on the supplied argument and write the resultant text to stdout.

(This is the equivalent of print from base)

writeR :: Render α => α -> Program τ () Source #

Pretty print the supplied argument and write the resultant text to stdout. This will pass the detected terminal width to the render function, resulting in appopriate line wrapping when rendering your value.


info :: Rope -> Program τ () Source #

Note a significant event, state transition, status; also used as a heading for subsequent debugging messages. This:

    info "Starting..."

will result in

13:05:55Z (00.112) Starting...

appearing on stdout. The output string is current time in UTC, and time elapsed since startup shown to the nearest millisecond (our timestamps are to nanosecond precision, but you don't need that kind of resolution in in ordinary debugging).

Since: 0.2.12

warn :: Rope -> Program τ () Source #

Emit a diagnostic message warning of an off-nominal condition. They are best used for unexpected conditions or places where defaults are being applied (potentially detrimentally).

     warn "You left the lights on again"

Warnings are worthy of note if you are looking into the behaviour of the system, and usually—but not always—indicate a problem. That problem may not need to be rectified, certainly not immediately.


For example, see Core.Program.Execute's trap_ function, a wrapper action which allows you to restart a loop when combined with forever. trap_ swollows exceptions but does not do so silently, instead using warn to log a warning as an information message. You don't need to do anything about the warning right away; after all the point is to allow your program to continue. If it is happening unexpectly or frequently, however, the issue bears investigation and the warning severity message will give you a starting point for diagnosis.

Since: 0.2.12

critical :: Rope -> Program τ () Source #

Report an anomoly or condition critical to the ongoing health of the program.

     critical "Unable to do hostname lookups"      -- Yup, it was DNS. It's always DNS.

The term "critical" generally means the program is now in an unexpected or invalid state, that further processing is incorrect, and that the program is likely about to crash. The key is to get the message out to the informational channel as quickly as possible before it does.

For example, an uncaught exception bubbling to the top the Program monad will be logged as a critical severity message and forceibly output to the console before the program exits. Your program is crashing, but at least you have a chance to find about why before it does.

You're not going to page your operations staff on these either, but if they're happening in a production service and it's getting restarted a lot as a result you're probably going to be hearing about it.

Since: 0.2.12


debug :: Rope -> Rope -> Program τ () Source #

Output a debugging message formed from a label and a value. This is like info above but for the (rather common) case of needing to inspect or record the value of a variable when debugging code. This:

    setProgramName "hello"
    name <- getProgramName
    debug "programName" name

will result in

13:05:58Z (03.141) programName = hello

appearing on stdout, assuming these actions executed about three seconds after program start.

debugS :: Show α => Rope -> α -> Program τ () Source #

Convenience for the common case of needing to inspect the value of a general variable which has a Show instance

debugR :: Render α => Rope -> α -> Program τ () Source #

Convenience for the common case of needing to inspect the value of a general variable for which there is a Render instance and so can pretty print the supplied argument to the log. This will pass the detected terminal width to the render function, resulting in appopriate line wrapping when rendering your value (if logging to something other than console the default width of 80 will be applied).