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Date: Sun, 16 Oct 2011 20:51:13 +0200
From: Matej Kosik <kosik@fiit.stuba.sk>
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Subject: [Caml-list] Ocaml: typing by name vs. typing by structure

Hello,

When I try to compile the following program:

   let f1 = function
     {re=a;im=b} ->
        print_float a;
        print_float b
    in
    f1 {re=10.0;im=20.0

I get an error message:

    File "tmp.ml", line 2, characters 2-13:
    Error: Unbound record field label re

I know how to "fix" the program to be compilable---I have to add

    type whatever = {re:float; im:float}

at the top.
Then type inference is able to figure out that "f1" has type "whatever -> unit".

Why did language designers chosen:
- typing by structure in case of tuples (tuples can be viewed as records with unlabled fields),
- typing by name in case of records.

Why wasn't typing by structure chosen both for tuples and for records?
If it were, then type of "f1" would be "{re=float;im=float}".

Best regards.
-- 
Matej Košík