package ppx_string

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Ppx extension for string interpolation

Install

Dune Dependency

Authors

Maintainers

Sources

ppx_string-v0.15.0.tar.gz
sha256=f33d4956792fd022c63f8eef4269fcbf77aa06402d7b17df80cb493482b71393

Description

Part of the Jane Street's PPX rewriters collection.

Published: 21 Mar 2022

README

README.org

This extension provides a syntax for string interpolation. Here is an example of
its features:

#+begin_src ocaml
let script_remotely (user : string) (host : string) (port : int) (script : string) =
  [%string "ssh %{user}@%{host} -p %{port#Int} %{Sys.quote script}"]
#+end_src

The above expression is equivalent to:

#+begin_src ocaml
let script_remotely (user : string) (host : string) (port : int) (script : string) =
  String.concat ""
    [ "ssh "
    ; user
    ; "@"
    ; host
    ; " -p "
    ; Int.to_string port
    ; " "
    ; Sys.quote script
    ]
#+end_src

Compared to =Printf.sprintf=:

#+begin_src ocaml
let script_remotely (user : string) (host : string) (port : int) (script : string) =
  sprintf "ssh %s@%s -p %d %s" user host port (Sys.quote script)
#+end_src

having the values inline instead of after the format string can make it easier
to understand the resulting string, and avoids the potential mistake of passing
arguments in the wrong order. This is truer the more format arguments there are.
On the other hand, some things are much easier with printf: pad numbers with
zeroes, pad strings on the right, display floats in a specific formats, etc.

Compared to manually writing something like =String.concat= version above,
ppx_string is shorter and can oftentimes be less error-prone (it's really easy
to forget whitespace after =ssh= or around =-p= in the explicit =String.concat=
version).

To emit the literal sequence =%{=, you can escape it as follows:

#+begin_src ocaml
[%string {|%{"%{"}|}]
#+end_src

To pad strings with spaces on the left, add an integer expression after a colon:

#+begin_src ocaml
[%string "%{col1#Int:term_width / 2}%{col2#:term_width/4}%{col3#:8}%{col4}"]
#+end_src

is equivalent to:

#+begin_src ocaml
  let pad str len =
    let pad_len = max 0 (String.length str - len) in
    let padding = String.make pad_len ' ' in
    padding ^ str
  in
  String.concat ""
    [ pad (Int.to_string col1) (term_width / 2)
    ; pad col2 (term_width / 4)
    ; pad col3 8
    ; col4
    ]
#+end_src
(note that the pad length can be dynamic, as with the format string "%*s")

Dependencies (5)

  1. ppxlib >= "0.23.0"
  2. dune >= "2.0.0"
  3. ppx_base >= "v0.15" & < "v0.16"
  4. base >= "v0.15" & < "v0.16"
  5. ocaml >= "4.08.0"

Dev Dependencies

None

Conflicts

None

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