Pattern matching left of the = sign is helpful for tuple, records and
destructuring variant types with a single constructors.

# let (x,y) = (1,2);;
> val x : int = 1
> val y : int = 2
> # type r = { x : int; y : int; };;
> type r = { x : int; y : int; }
> # let {x=a;y=b} = {x=1;y=2};;
> val a : int = 1
>
>> val b : int = 2
>
> # let {x=a} = {x=1;y=2};;
>
> val a : int = 1
>
# type point = Point of int * int;;
> type point = Point of int * int
> # let Point (x,_) = Point(1,2);;
> val x : int = 1
>

--
nL

On Fri, Mar 18, 2016 at 2:04 PM, Mr. Herr <misterherr@freenet.de> wrote:

> 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.
>