Janet 1.16.1-87f8fe1 Documentation
(Other Versions: 1.15.0 1.13.1 1.12.2 1.11.1 1.10.1 1.9.1 1.8.1 1.7.0 1.6.0 1.5.1 1.5.0 1.4.0 1.3.1 )


The janet binary comes with a built in linter as of version 1.16.1. This allows the compiler to emit warnings that don't always need to stop compilation. Linting is enabled by default at the command line and can be adjusted with the -x and -w flags.

Linting messages from the compiler contain a linting level, a message, and source code location. There are 3 default levels of linting messages:

Note that the levels of lints may seem backwards, as the levels of the lint are from the perspective of the programmer. For example, a "picky" lint message that was warning the user about a matter of style would only show up if the programmer had `:strict` mode enabled for linting. You can also use the priority integers directly for lint levels.


There is also support for deprecation messages in the compiler to trigger lint warnings if a user attempts to use a deprecated binding. Deprecation here is the process of signaling to users that a function or other binding should not longer be used for new code. Deprecation levels are the same as the linting levels.

(defn old-fn 
 "This function is deprecated"
 (* 2 x))

(defn older-fn
 {:deprecated :relaxed}
 "This function is deprecated, and we really don't want you to use it."
 (* 2 x))

Lint Warnings and Errors

Whenever Janet code is being executed by the run-context function, there are lint thresholds for both errors and warnings. If a lint message has a level equal to or below the error threshold, it will be converted into a compiler error. Otherwise, if the lint message has a level equal to or below the warning threshold, a warning will be printed to stderr. Finally, lints above both the error and warning threshold will be ignored.

The error threshold can be set with (setdyn :lint-error error-threshold), and the warning threshold can similarly be set with (setdyn :lint-warn warn-threshold), where threshold values are any of the lint levels (keywords) or an integer. By default, the error threshold is 0 (no lints will cause an error), and the warn threshold is 2 (relaxed and normal lints will cause a warning to print, but strict lints will be ignored). These threshold can also be set at the command line with the -w and -x flags for the warn and error thresholds respectively.

Macro Linting

Inside macros, Janet exposes an array used to collect lint messages. A function maclintf is provided to help construct error messages if some issue occurs inside a macro. maclintf can be used to check arguments and assert that certain invariants are held when invoking a macro.

(defmacro my-plus
 "Do addition and hint that the first argument should be a constant number."
 [x y]
 (unless (number? x)
  (maclintf :normal "my-plus should be invoked with a numeric constant as x, got %v" x))
 ~(,+ ,x ,y))

Use of maclintf over other methods of reporting warnings is preferred as the programmer has the ability to customize how the handle different levels of lint messages. For fatal errors inside the macro, use the error function as usual.