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.