# Flycheck versus Flymake

This article compares Flycheck to the built-in Flymake mode. It does not consider third-party extensions such as flymake-easy, but references them at appropriate places.

We aim for this comparison to be fair and comprehensive, but it may contain stale information. Please report any inaccuracy you might find, and feel free to edit this page and improve it.

Note

This comparison was updated at the time of the Emacs 26.1 release, which contains an overhaul of Flymake. If you are using Emacs 25.3 or below, you can still access the comparison between Flycheck and the legacy Flymake here.

## Overview

This table gives an overview of the differences and similarities between Flycheck and Flymake. The rest of this page describes each point in more detail.

| Flycheck | Flymake
--- | --- | ---
Supports Emacs versions | 24.3+ | 26.1+
Built-in | no | yes
Supported languages | 100+ built-in, 200+ w/ 3rd-party | 10 built-in, 50+ w/ 3rd party
Automatic syntax checking | built-in | manual
Check triggers | save, newline, change, buffer switch | save, newline, change
Asynchronous checking | yes, always | yes, for some modes
Automatic syntax checker selection | by major mode and custom predicates | no
Multiple syntax checkers per buffer | yes (configurable chain) | yes (all at once)
Definition of new syntax checkers | single declarative macro | arbitrary function 1
Configuration debugging | built-in (C-c ! v) | none
Error identifiers | yes | no
Error explanations | yes | no
Error parsing helpers | for regexp, JSON and XML | none
Fringe icons for errors | yes | yes
Error highlighting | faces, brackets, mixed | faces only
Error indicators | fringes (incl HiDPI), margins | fringes only
Error message display | tooltip, echo area, fully customizable (e.g. tooltip, popup w/ 3rd party packages) | tooltip, echo area
List of all errors | yes; filterable by error level | yes

## Detailed review

### Relation to Emacs

Flymake has been part of GNU Emacs since GNU Emacs 22. As such, contributions to Flymake are subject to the FSF policies on GNU projects. Most notably, contributors are required to assign their copyright to the FSF.

Flycheck is not part of GNU Emacs. However, it is free software as well, and publicly developed on the well-known code hosting platform Github. Contributing to Flycheck does not require a copyright assignment, only an explicit agreement that your contributions will be licensed under the GPL.

### Automatic syntax checking

Flymake is not enabled automatically for supported languages. It must be enabled for each mode individually, or by, e.g., adding to a hook that enables it for all `prog-mode` buffers. If no backends for the major mode are available, Flymake will non-intrusively tell you in the modeline.

Flycheck provides a global mode `global-flycheck-mode` which enables syntax checking in every supported language, where it is safe to do so (remote and encrypted buffers are excluded by default).

### Syntax checkers

#### Supported languages

Flymake comes with support for Emacs Lisp, Ruby (`ruby` for syntax check and `rubocop` for lints), Python and Perl. In addition, backends written for the legacy Flymake are compatible with the new implementation.

Flycheck provides support for over 50 languages with over 100 syntax checkers, most of them contributed by the community.

#### Definition of new syntax checkers

Flymake backends are single functions which report diagnostics to a callback function given as argument.

Flycheck provides a single function `flycheck-define-checker` to define a new syntax checker. This function uses a declarative syntax which is easy to understand even for users unfamiliar with Emacs Lisp. In fact, most syntax checkers in Flycheck were contributed by the community.

For example, the Perl checker in Flycheck is defined as follows:

    (flycheck-define-checker perl
      "A Perl syntax checker using the Perl interpreter.

    See URL `http://www.perl.org'."
      :command ("perl" "-w" "-c" source)
      :error-patterns
      ((error line-start (minimal-match (message))
              " at " (file-name) " line " line
              (or "." (and ", " (zero-or-more not-newline))) line-end))
      :modes (perl-mode cperl-mode))

The whole process is described in Adding a syntax checker to Flycheck.

#### Customization of syntax checkers

Flymake does not provide built-in means to customize syntax checkers. Instead, when defining a new syntax checker the user needs to declare customization variables explicitly and check their value in the init function.

Flycheck provides built-in functions to add customization variables to syntax checkers and splice the value of these variables into the argument list of a syntax checking tool. Many syntax checkers in Flycheck provide customization variables. For instance, you can customize the enabled warnings for C with `flycheck-clang-warnings`. Flycheck also tries to automatically find configuration files for syntax checkers.

#### Executables of syntax checkers

Flymake does not provide built-in means to change the executable of a syntax checker.

Flycheck defines a variable to set the path of a syntax checker tool for each defined syntax checker and provides the interactive command `flycheck-set-checker-executable` to change the executable used in a buffer. The process used to locate checker configuration files can also be customized using `flycheck-locate-config-file-functions`, allowing you to store your personal checker configuration files in your `.emacs.d` folder.

### Syntax checker selection

Flymake runs all functions added to the `flymake-diagnostic-functions` hook.

Flycheck uses the major mode and checker-specific predicates to

  automatically select a syntax checker.

#### Custom predicates

Flymake may allow for backends to implement custom logic to decide whether to run the check or not. There are no easily-defined predicate functions.

Flycheck supports custom predicate functions. For instance, Emacs uses a single major mode for various shell script types (e.g. Bash, Zsh, POSIX Shell, etc.), so Flycheck additionally uses a custom predicate to look at the value of the variable `sh-shell` in Sh Mode buffers to determine which shell to use for syntax checking.

#### Manual selection

Flymake users may manually select a specific backend by overriding the value of the backends list.

Flycheck provides the local variable `flycheck-checker` to explicitly use a specific syntax checker for a buffer and the command `flycheck-select-checker` to set this variable interactively.

#### Multiple syntax checkers per buffer

Flymake will use all the backends added to the `flymake-diagnostic-functions` hook to check a buffer; all backends are started at the same time, but errors are reported in the buffer as soon as a backend returns them. Backends can also be written to first report errors for the visible region of the buffer, and collect errors for hidden regions later.

Flycheck can also apply multiple syntax checkers per buffer, but checkers run in sequence rather than concurrently. For instance, Flycheck will check PHP files with PHP CLI first to find syntax errors, then with PHP MessDetector to additionally find idiomatic and semantic errors, and eventually with PHP CheckStyle to find stylistic errors. The user will see all errors reported by all of these tools in the buffer. These checker-chains are configurable (see Configuring checker chains), so it’s possible to run an advanced style checker only if a basic syntax checker returned no errors (this avoids accumulating too many false positives and improves performance).

### Errors

#### Error identifiers

Flymake does not include special treatment for error identifiers.

Flycheck supports identifiers for different kinds of errors, if a syntax checker provides these. The identifiers appear in the error list and in error display, and can be copied independently, for instance for use in an inline suppression comment or to search the web for a particular kind of error.

#### Error explanations

Some Flycheck checkers can use error identifiers to provide error explanations in an help buffer (see `flycheck-explain-error-at-point`).

#### Error indicators

Both Flymake and Flycheck indicate errors in the buffer (using overlays) and in the fringes. Flycheck includes fringe bitmaps for HiDPI screens, and also supports displaying indicators in the margins instead of the fringes (this behavior can be customized using `flycheck-indication-mode`, and `flycheck-highlighting-mode`).

#### Error parsing

Flymake lets backend parse error messages from tools. There are no built-in helpers for defining error patterns, or for parsing JSON or XML formats.

Flycheck checkers can use regular expressions as well as custom parsing functions. The preferred way to define a checker is to use the `rx` syntax, extended with custom forms for readable error patterns. Flycheck includes some ready-to-use parsing functions for common output formats, such as Checkstyle XML, or JSON interleaved with plain text.

#### Error message display

Flymake shows error messages in a tool tip if the user hovers the mouse over an error location, or in the echo area if the user navigates to the error with `flymake-goto-next-error`.

Flycheck shows error message in tool tips as well, and also displays error messages in the echo area if the point is at an error location. This feature is fully customizable via `flycheck-display-errors-function`, and several extensions already provide alternative way to display errors.

Footnotes

1

  flymake-easy provides a function to define a new syntax checker, which sets all required variables at once.