Ocaml record matching

Question

Given a basic record

type t = {a:string;b:string;c:string}

why does this code compile

let f t = match t with
    {a;b;_} -> a

but this

let f t = match t with
    {_;b;c} -> b

and

let f t = match t with
    {a;_;c} -> c

does not? I'm asking this out of curiosity thus the obvious useless code examples.

Answer 1

The optional _ field must be the last field. This is documented as a language extension in Section 7.2

Here's the production for reference:

pattern ::= ...  
    ∣ '{' field ['=' pattern]  { ';' field  ['=' pattern] }  [';' '_' ] [';'] '}'

Answer 2

Because the latter two examples are syntactically incorrect. The syntax allows you to terminate your field name pattern with the underscore to notify the compiler that you're aware, that there are more fields than you are trying to match. It is used to suppress a warning (that is disabled by default). Here is what the OCaml manual says about it:

Optionally, a record pattern can be terminated by ; _ to convey the fact that not all fields of the record type are listed in the record pattern and that it is intentional. By default, the compiler ignores the ; _ annotation. If warning 9 is turned on, the compiler will warn when a record pattern fails to list all fields of the corresponding record type and is not terminated by ; _. Continuing the point example above,

If you want to match to a name without binding it to a variable, then you should use the following syntax:

{a=_; b; c}

E.g.,

let {a=_; b; c} = {a="hello"; c="cruel"; b="world"};;
val b : string = world
val c : string = cruel