package ppx_string_interpolation

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String interpolation PPX preprocessor.

Published: 28 Jan 2021


PPX rewriter that enables string interpolation.



This PPX implements string interpolation with embedded expressions, thus enabling simple and powerful templates for code generation and pretty printing. In contrast to printf, string interpolation does not separate embedded variables and context, which is important for medium-size strings (several lines).

Quick Start

Plugin uses bash-style syntax for string interpolation and the type of variables and expressions is assumed to be string by default:

let name = "world" in
    [%string "Hello $name!"]

Embedded expressions should be enclosed in parentheses:

let hello = "Hello" and world = "world" in
    [%string "$(hello ^ \" \" ^ world)!"]

Non-string types

Since this PPX run before type inference and type checking, rewriter should have the information about the types of embedded values via Printf.printf format specifiers:

let a = 1 and b = 1.0 in
    [%string {|We know, that %d$a == %f$b is %b$(a = int_of_float b)!|}]

Advanced usage

The extension is essentially syntax sugar for the Printf.sprintf function. The format is assumed to be anything between % and the next $. The expression string is determined by searching for balanced parentheses.

Both $ and % symbols can be escaped, i.e. double % and double $ result in single characters:

[%string "This is $$ and %%!"] = "This is $ and %!"

If the expression is commented out, the PPX dumps the comment with format, without checking for the type of the format:

[%string "The first pythagorean triple: %d$(3), 4, %d$(*5?*)"] =
    "The first pythagorean triple: 3, 4, %d$(*5?*)"


To build the project use dune build.


To install the package run dune install.


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Please read the LICENSE file.

Code of Conduct

This project has adopted a Code of Conduct. If you have any concerns about the Code, or behavior which you have experienced in the project, please contact us at

Security Vulnerability Reporting

If you believe you have identified a security vulnerability in this project, please send email to the project team at, detailing the suspected issue and any methods you've found to reproduce it.

Please do NOT open an issue in the GitHub repository, as we'd prefer to keep vulnerability reports private until we've had an opportunity to review and address them.

Dependencies (4)

  1. sedlex >= "2.0"
  2. ppxlib >= "0.18.0"
  3. ocaml >= "4.04.1"
  4. dune >= "1.7"

Dev Dependencies


Used by (1)

  1. lablqml >= "0.7"