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A programming language for writing libraries and other reusable things
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Reuse is a general-purpose programming language designed to compile to other programming languages. This dramatically increases the number of platforms and languages that libraries written in Reuse support.

The language is strongly typed with a complete and sound type system and decidable type inference. The language does not support side-effects by design and all data is immutable. Any input/output or other kinds of side-effects can be performed by the host language.



This is what a factorial function looks like in Reuse:

(def factorial (n)
     (match (< n 2)
            True   1
            False  (* n (factorial (- n 1)))))

Another example involving algebraic data-types:

(typ (list a) (Cons a (list a))

(def reduce (f initial list)
     (match list
            Empty       initial
            (Cons x xs) (reduce f (f x initial) xs)))

For more examples please look through the minimal/specification and extended/specification directories. Also see the getting started section.



The Reuse compiler is distributed using a Homebrew tap. Install it using the following commands:

brew tap redien/reuse
brew install --HEAD reuse


Clone the repository and symlink [repository path]/reusec to the appropriate directory in your PATH.


Windows support is currently not implemented.

Getting Started

Let's create a library for the factorial function from the previous section by creating a Reuse source file called factorial.reuse.

(def factorial (n)
     (match (< n 2)
            True   1
            False  (* n (factorial (- n 1)))))

Once we've saved the source file we need to compile it using the Reuse compiler called reusec.

$ reusec --language ocaml --output factorial.reuse

This will compile the library we just wrote into an OCaml source file named We can compile to all supported languages using the same Reuse source file. See the usage section for a list of supported languages and the different compiler options.

Let's write a small OCaml program that will use our library to calculate the factorial of 10. We write it in a file we'll name

Printf.printf "%ld\n" (Factorial.factorial 10l)

Reuse has an integer type of 32 bits while OCaml's default integers vary in size. This is why we need to represent the constant 10 as 10l to tell the OCaml compiler to treat it as a 32 bit integer.

We can now compile the example using the OCaml compiler.

$ ocamlopt -c
$ ocamlopt -c
$ ocamlopt -o factorial factorial.cmx main.cmx

And finally if we run the compiled program we should get the correct result for the factorial of 10.

$ ./factorial


Integer manipulation

The int32 type is a 32-bit signed integer.

(+ 1 2)    3
(- 1 2)    -1
(* 1 2)    2
(/ 1 2)    0
(% 1 2)    1

(= 1 2)    False
(< 1 2)    True
(> 1 2)    True
(>= 1 2)   False
(<= 1 2)   True

Boolean operators

(and True False)    False
(or True False)     True
(not True)          False

Function Definition

(def identity (x)


Usage: reusec [flags] --output [OUTPUT FILE] [FILE]...

Compiler for the Reuse programming language

       --minimal          Source language should be treated as Minimal Reuse
       --nostdlib         Do not include the standard library
       --language [LANG]  Target language to compile to.
                          Can be ocaml or minimal.
       --output [FILE]    Write output to FILE

Design Rationale

  • Keeping the language pure
  • Keeping the language as small as possible
    • Pro: Reduces the work of implementing new language targets and tools
    • Pro: Fewer features to learn
    • Con: Not as expressive as it could be
  • No let expression
    • Pro: let generalization is not frequently used.
    • Pro: A match expression can work just as well as a let expression
    • Con: Might encourage breaking up functions that would be better understood as just one

Minimal and extended

Reuse has a minimal subset making it trivial to write a translator for a new language.

The extended language gives us more convenience and compiles down to the minimal subset.


Use the prepared docker image to launch a bash shell with the necessary tools:

docker run --rm -it -v $PWD:/home/opam/reuse-lang redien/reuse-lang-dev-env


  • Re-implement a javascript backend to get a debugger for free
  • Do not rely on string operations other than equality when parsing
  • Generate a symbol table after parsing s-expressions.
  • Implement modules by parsing other files and adding them to the symbol table with separate IDs
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