if-changed

if-changed is a command-line utility that checks for "if-changed" and "then-change" comments in a repository diff and errors if dependent files need changes.

Installation

cargo install if-changed

Usage

Usage: if-changed [OPTIONS] [PATTERNS]...

Arguments:
  [PATTERNS]...
          Git patterns defining the set of files to check. By default, this will be all changed files between revisions.

          This list follows the same rules as [`.gitignore`](https://git-scm.com/docs/gitignore) except relative paths/patterns are always matched against the repository root, even if the paths/patterns don't contain `/`. In particular, a leading `!` before a pattern will reinclude the pattern if it was excluded by a previous pattern.

Options:
      --from-ref <FROM_REF>
          The revision to compare against. By default, HEAD is used

          [env: PRE_COMMIT_FROM_REF=]

      --to-ref <TO_REF>
          The revision to compare with. By default, the current working tree is used

          [env: PRE_COMMIT_TO_REF=]

  -h, --help
          Print help (see a summary with '-h')

  -V, --version
          Print version

To use with pre-commit, add the following to your .pre-commit-config.yaml:

repos:
  - repo: https://github.com/mathematic-inc/if-changed
    rev: v0.3.1
    hooks:
      - id: if-changed

Motivating example

Suppose you have the following:

// lib.rs
enum ErrorCode {
    A,
    B,
    C,
}

// lib.ts
const enum ErrorCode {
  A,
  B,
  C,
}

Typically, to synchronize these enums, a common approach is to extract the enum values into a "source-of-truth" file. This often requires significant effort to generate the enums using the build system or a script, and to ensure everything works correctly. If the job is a one-off, the costs heavily outweigh the benefits.

This is where if-changed comes in. Instead of the above, suppose we have:

 // lib.rs
+// if-changed(ecrs)
 enum ErrorCode {
     A,
     B,
     C,
 }
+// then-change(lib.ts:ects)

 // lib.ts
+// if-changed(ects)
 const enum ErrorCode {
   A,
   B,
   C,
 }
+// then-change(lib.rs:ecrs)

Once this is commited, the next time lib.rs (or lib.ts) is changed in the lines surrounded by "if-changed" and "then-change", if-changed will error if the other file (referenced in the "then-change" comment) does not have any changes in the corresponding named block.

[!TIP]

If you just want to assert that any change in a file is ok, then just reference the file without the name. For example,

 // lib.ts
 // if-changed(ects)
 const enum ErrorCode {
   A,
   B,
   C,
 }
-// then-change(lib.rs:ecrs)
+// then-change(lib.rs)

File lists

If a block needs to specify several files, you can use commas and/or newlines to separate paths/patterns. For example,

// then-change(foo/bar, baz)

/// OR

// then-change(
//   foo/bar
//   bar
// )

These lists follow the same rules as .gitignore, with the exception that relative paths/patterns are always matched against the file in which they are written, even if the paths/patterns don't contain /. Use a starting / to match the pattern against the repository root, e.g. /*/bar.

Long paths

If a path is too long, you can use a shell continuation \ to split it across multiple lines. For example, for the path this/is/a/really/long/path/to/some/very/far/away/file, you can do

// then-change(
//   this/is/a/really/long/path/to/some/very/far/ \
//   away/file
// )

Disabling if-changed

To disable if-changed for a specific file during a commit, add Ignore-if-changed: <path>, ... -- [REASON] to the commit footer. Here, <path> should be the file path. In general, <path> can be any pattern allowed by fnmatch.

[!NOTE]

If you want to disable if-changed when diffing the working tree, you can execute if-changed with the following:

if-changed '*' !<path-or-pattern>

where <path-or-pattern> is the path/pattern you want to ignore.

Contributing

Contributions to if-changed are welcome! Please submit a pull request or create an issue in the GitHub repository.