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To: caml-list@inria.fr
Subject: pattern matching and records vs tuples
From: Yoann Padioleau <padator@wanadoo.fr>
Date: Tue, 14 Apr 2009 09:12:19 -0500
Message-ID: <87skkbuxx8.fsf@aryx.cs.uiuc.edu>
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X-Spam: no; 0.00; compiler:01 compiler:01 foo:01 ocaml:01 'foo':01 ocaml:01 foo:01 tuples:01 tuples:01 int:01 int:01 tuple:02 exhaustive:03 let:03 let:03 


Hi, 

I've found that while records provide advantages over tuples, 
they also have disadvantages when it comes to evolution issues. 
If I decide to evolve code using a tuple type, for instance adding
new information and so extend a 4-uple in a 5-uple, then the compiler
will tell me all the places that I need to update, which is good.
If I use records instead, and have 4 fields, and I want to add again
some new information in a new field, then the way the compiler works
right now will not help me at all. E.g with this code 

  type record = { 
    field1: int;
    field2: int;
  }
  let foo = function 
   { field1 = v1; field2 = v2 } -> ...


If I extend record with a new field   field3, then ocaml will
not warn me and tell me to modify also the 'foo' function :(


So, would it be possible to extend the ocaml compiler with a simple
feature that let the programmer tell the compiler when he use 
an "exhaustive" pattern with record, e.g.

 let foo = function
 { field1 = v1; field2 = v2; NOTHING_ELSE} -> 

(the inverse of a '...')