Original bug ID: 4172
Reporter: acone
Assigned to: @pierreweis
Status: closed (set by @alainfrisch on 2014-02-17T15:44:20Z)
Resolution: fixed
Priority: normal
Severity: feature
Category: ~DO NOT USE (was: OCaml general)
Related to: #5525

Bug description

In OCaml, the following causes problems:

type t1 = {a:int; b:int}
type t2 = {a:string; c:string}
let make_t1() = {a=5; b=10}

It's clear to our eyes that make_t1 should return a t1, not a t2, but the compiler assumes that it returns a t2. It does this because I've used a label that's bound in t2 (namely 'a'), and t2 was declared later. So when I run this code I get a type error.

I understand that the compiler does not try to infer which of t1 or t2 is meant because this requires exponential time. I admit that exponential time is unacceptable, but I feel there are better solutions than simply defaulting to the most recently declared type.

1. (the slighly less crappy solution)
   warn the user whenever a parameter in a record type declaration shadows a parameter in a previous record type declaration.

2. (the solution I'd prefer)
   whenever an expression's type is not uniquely specified by parameter names, require the user to state it's type explicitly.
   For example, the following would be allowed:

type t1 = {a:int; b:int}
type t2 = {a:string; b:string}
let print_t1 (x : t1) = printf "(%d, %d)" x.a x.b

But the following would produce an error message like "Expression type can not be determined from parameter names. Please state its type explicitly.":

type t1 = {a:int; b:int}
type t2 = {a:string; b:string}
let print_t1 x = printf "(%d, %d)" x.a x.b

It should not insist, as it currently does, that I wanted x to be a t2.

Additional information

As it stands, the lack of this feature is a reason I sometimes avoid Caml. The standard solution--to prefix each parameter with the type's name--results in ugly, unreadable code.

The fix I'm proposing would not affect the computational complexity of type inference. It would simply require explicit typing for expressions that should need it.