ppx_match_literals

ppx_match_literals is an OCaml extension written using ppxlib that allows for variables to be placed in match patterns and have them be interpreted as their literal value. Consider the following simple example usage:

match%literals var_1 with
| (_, [%lit var_2]) -> expr

How to use as literals

To specify that a match expression will have patterns that require this extension, create the match statement using the extension %literals with infix syntax; ie, use match%literals instead of match when going to use literal variables in the cases. Inside a match%literals expression, literal variables can be specified in a case pattern using the %lit extension with the variable name as the parameter. Usage would look like [%lit var_name_here].

How it works

This extension traverses the patterns of all match%literals expressions and replaces the extension nodes representing [%lit id] patterns with wildcard patterns. An extra when guard is appended for every wildcard that needs to be matched to a variable. To avoid namespace collision, wildcards are given the name ppx_match_literals_lit_* where "*" is replace with the name of the variable to match.

Currently supports recursing into the following patterns

• tuple
• construct
• record
• alias
• or

Example usage

Before desugaring, using extension

let hello = "hello" in
let hi = "hi" in
match%literals ("hello", "abc", "hi") with
| ([%lit hello], "abc", _) when boolean_1 -> 1
| (_, "123", [%lit hi]) -> 2
| ([%lit hello], "abc", [%lit hi]) -> 3
| _ -> 4

After desugaring

let hello = "hello" in
let hi = "hi" in
match ("hello", "abc", "hi") with
| (ppx_match_literals_lit_hello, "abc", _) when 
    boolean_1 && ppx_match_literals_lit_hello = hello -> 1
| (_, "123", ppx_match_literals_lit_hi) when ppx_match_literals_lit_hi = hi -> 2
| (ppx_match_literals_lit_hello, "abc", ppx_match_literals_lit_hi) when 
    ppx_match_literals_lit_hello = hello && ppx_match_literals_lit_hi = hi-> 3
| _ -> 4

More complicated usage can be found in example.ml.

Acknowledgement

Large thanks to jacketsj for the initial idea and directional advice for implementation.