From: octachron <octa@polychoron.fr>
To: caml-list@inria.fr
Subject: Re: [Caml-list] what does let () = ... define?
Date: Fri, 18 Mar 2016 20:00:50 +0100 [thread overview]
Message-ID: <56EC5062.5070408@polychoron.fr> (raw)
In-Reply-To: <56EC4318.2020701@freenet.de>
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Hi,
Maybe defining is not the right word here. An alternative explanation
would be that "let" binds variables in the pattern on the left in order
to match the expression on the right. For instance, the following exotic
expression:
# type alt = Left of int | Right of int
# let (Left a|Right a) = Left 0;;
- : val a: int
is valid and binds the variable "a" to the value 0.
Since there is no variable to be bound in "()" or any constructor
without argument,
#let () = ();;
just computes the expression on the right and does not bind anything on
the left.
Note that in some way, the toplevel is already telling you that, since
# let () = ();;
has no output compared to
# ();;
- : unit = ()
which prints the value of the constructor ().
− octachron
Le 03/18/16 19:04, Mr. Herr a écrit :
> Hi,
>
> in a small presentation of OCaml (Linux User Group Bremen) I got some
> interesting questions, and
> trying to answer I noticed I took something for granted that was not
> fully understood.
>
> Looking at this in the toplevel:
>
> # let () = () ;;
> # () ;;
> - : unit = ()
> # let _ = () ;;
> - : unit = ()
> # let None = None;;
> Warning 8: this pattern-matching is not exhaustive.
> Here is an example of a value that is not matched:
> Some _
> #
>
> ... the question is: okay, pattern matching left of the equal sign,
> but what does it define?
> It defines unit and None in the environment, and then that value is
> just sitting there?
>
> /Str.
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next prev parent reply other threads:[~2016-03-18 19:00 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-03-18 18:04 Mr. Herr
2016-03-18 18:26 ` Nick Lucaroni
2016-03-18 18:52 ` Christoph Höger
2016-03-18 22:06 ` Nicolas Barnier
2016-03-18 19:00 ` octachron [this message]
2016-03-18 19:42 ` Stefan Monnier
2016-03-18 21:29 ` Raphaël Proust
2016-03-18 21:54 ` Mr. Herr
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