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Dead values/types, exception, and termination analysis for OCaml/ReScript


Dune Dependency






Experimental analyses for ReScript/OCaml/Reason: for globally dead values/types, exception analysis, and termination analysis.

Published: 08 Jun 2022



Program analysis for ReScript and OCaml projects targeting JS (ReScript) as well as native code (dune):

  • Globally dead values, redundant optional arguments, dead modules, dead types (records and variants).

  • Exception analysis.

  • Termination.


Early release. While the core functionality is reasonably stable, the CLI and annotations are subject to change. However, this is a tiny surface at the moment.


The rest of this document describes the dead code analysis. For the Exception Analysis, build instructions are the same, and the command-line invocation is different.

Build and run on existing projects using the Build and Try instructions below. The analysis uses .cmt[i] files which are generated during compilation, so should be run after building your project. Remember to rebuild the project before running again.

CLI for ReScript projects

# dead code analysis
reanalyze.exe -dce

# exception analysis
reanalyze.exe -exception

The requirement is that bsconfig.json can be found by walking up the current directory.

CLI for native projects

# dead code analysis
reanalyze.exe -dce-cmt root/containing/cmt/files

# exception analysis
reanalyze.exe -exception-cmt root/containing/cmt/files

Subdirectories are scanned recursively looking for .cmt[i] files.

The requirement is that the current directory is where file paths start from. So if the file path seen by the compiler is relative src/core/ then the current directory should contain src as a subdirectory. The analysis only reports on existing files, so getting this wrong means no reporting.

DCE reports

The dead code analysis reports on globally dead values, redundant optional arguments, dead modules, dead types (records and variants).

A value x is dead if it is never used, or if it is used by a value which itself is dead (transitivity). At the top level, function calls such as Js.log(x), or other expressions that might cause side effects, keep value x live.

An optional argument ~argName to a function is redundant if all the calls to the function supply the argument, or if no call does.

A module is considered dead if all the elements defined it in are dead.

The type analysis repots on variant cases, and record labels.

  • A variant case | A(int) is dead if a value such as A(3) is never constructed. But it can be deconstructed via pattern matching | A(n) => ... or checked for equality x == A(3) without making the case A live.

  • A record label x in type r = {x:int, y:int} is dead if it is never read (by direct access r.x or pattern matching | {x:n, y:m} => ...). However, creating a value let r = {x:3, y:4} does not make x and y live. Note that reading a value r does not make r.x or r.y live.

While dead values can be removed automatically (see below), dead types require a bit more work. A dead variant case requires changing the type definition, and the various accesses to it. A dead record label requires changing the type definition, and removing the label from any expressions that create a value of that type.

DCE: controlling reports with Annotations

The dead code analysis supports 2 annotations:

  • @dead suppresses reporting on the value/type, but can also be used to force the analysis to consider a value as dead. Typically used to acknowledge cases of dead code you are not planning to address right now, but can be searched easily later.

  • @live tells the analysis that the value should be considered live, even though it might appear to be dead. This is typically used in case of FFI where there are indirect ways to access values. In case of bucklescript projects using genType, export annotations immediately qualify values as live, because they are potentially reachable from JS.

The main difference between @dead and @live is the transitive behaviour: @dead values don't keep alive values they use, while @live values do.

Several examples can be found in examples/deadcode/src/DeadTest.res

Command-line Interface

CLI -suppress

Takes a comma-separated list of path-prefixes. Don't report on files whose path has a prefix in the list (but still use them for analysis).

reanalyze.exe -suppress one/path,another/path

CLI -unsuppress

Takes a comma-separated list of path-prefixes. Report on files whose path has a prefix in the list, overriding -suppress (no-op if -suppress is not specified).

reanalyze.exe -unsuppress one/path,another/path/File.res

CLI -debug

Print debug information during the analysis

reanalyze.exe -debug ...

Add annotations automatically

This overwrites your source files automatically with dead code annotations:

reanalyze.exe -write ...

Remove code automatically (not interactively)

There's a dead code ppx (values only, not types) in this repository. It can be used to automatically remove code annotated @dead, as if it had been commented out. Can be used after adding annotations automatically. The combination of automatic annotation and automatic elimination is a form of automatic dead code elimination. For projects that use a library, or that in general have code which is dead only temporarily. There's obviously a level of risk in doing this automatic elimination. The safety net you can rely on is that the code must still compile.

CLI -live-names

This automatically annotates @live all the items called foo or bar:

-live-names foo,bar

CLI -live-paths

This automatically annotates @live all the items in file Hello.res:

-live-paths Hello.res

This automatically annotates @live all the items in the src/test and tmp folders:

-live-paths src/test,tmp

Configuration via bsconfig.json

The -config option can be used to read the configuration from bsconfig.json: to set what analyses should be run, as well as suppress and unsuppress configuration.

Example configuration inside bsconfig.json:

  "reanalyze": {
    "analysis": ["dce", "exception"],
    "suppress": ["src/ToSuppress.res"],
    "unsuppress": ["this", "that"]

This is equivalent to adding -dce -exception -suppress src/ToSuppress.res -unsuppress this,that to the command line in place of -config. Note that the options are additive, so it's possible to use e.g. -config -exception to add exception analysis on top of what the configuration does.

Install with npm for ReScript projects

npm add --save-dev reanalyze

Build From Sources

Build for ReScript

opam install dune
npm run build406
# _build/default/src/Reanalyze.exe

Build for OCaml native projects using dune

opam install dune
dune build
# _build/default/src/Reanalyze.exe

Try it

ReScript Projects (JS output)

npm run build # or whatever command to build the project
npm add --save-dev reanalyze
npx reanalyze -dce

Native Projects (OCaml using dune)

Make sure that dune builds both .cmt and .cmti files by enabling bytecode compilation. This is normally done by adding (modes byte exe) to the executable stanza in your dune file (see

This project is itself written in OCaml and can be analyzed as follows.

dune build
./_build/default/src/Reanalyze.exe -suppress src/compiler-libs-406 -dce-cmt _build

Dependencies (3)

  1. cppo build
  2. ocaml >= "4.08" & < "4.15"
  3. dune >= "2.0"

Dev Dependencies


Used by