Original bug ID: 5528
Reporter: @alainfrisch
Assigned to: @alainfrisch
Status: closed (set by @xavierleroy on 2017-09-24T15:31:52Z)
Resolution: fixed
Priority: normal
Severity: feature
Fixed in version: 4.03.0+dev / +beta1
Category: ~DO NOT USE (was: OCaml general)
Related to: #5525 #6374
Monitored by: pierpa @bobzhang @gasche @rixed @diml pveber bobot Bardou Camarade_Tux lavi @jmeber @yallop @hcarty yminsky @Chris00 @yakobowski

Bug description

OCaml allows n-ary constructors for sum types. Instead of relying on position, it would be convenient to name the fields. Of course, one can use records, but this requires an extra type declaration and has some runtime overhead.

I've started a new branch (constructors_with_record) in the SVN to allow naming arguments of constructors, with the same syntax as records.

Example:

type t =
| A of {msg: string; foo:int}
| B of string
| C

let f = function
| A {msg; _} | B msg -> msg
| C -> ""

GADTs and exceptions are supported. It is possible to define mutable fields, but there is currently no way to mutate them. Polymorphic fields are not supported.

Note that this proposal also gives a way to disambiguate record field names:

type a = A of {foo: string; bar: int}
type b = B of {foo: int; baz: bool}
let f (A{foo; _}) = foo
let g (B{foo; _}) = foo

To support mutation and field overriding, I was thinking of a syntax like:

type t = A of {mutable l: int} | B of {x:int; y:int}

match ... with
| A r -> r.l <- r.l + 10
| ...

match ... with
| B r -> B {r with x = r.y; y = r.x}
| ...

Binding (directly like above or with an alias pattern) the "record" argument creates a special value (special in the same way that "self" or "ancestor" variables in objects are special) which can only be used in the following context:

• field projection: r.l
• field assignment: r.l <- e
• overidding: B {r with ...}

File attachments

• patch_encoding.diff