Re: To be or not to be - these are the options for Option.map

On 6 September 2014 17:02, [email protected] [ocaml_beginners]
<[email protected]> wrote:
> However, looking in ~/.opam/system/lib/core_kernel/option.mli reveals no definition of the map function. The closest match is "val map2 : 'a t -> 'b t -> f:('a -> 'b -> 'c) -> 'c t".
>
> How can the code compile just fine when the 'map' definition isn't there?

The signature for the Option module has an 'include' line:

   include Monad.S with type 'a t := 'a t
   https://github.com/janestreet/core_kernel/blob/5aa53f/lib/option.mli#L8

This line means, roughly, "copy the contents of Monad.S at this
point".  The Monad.S signature has a map function

   val map : 'a t -> f:('a -> 'b) -> 'b t
   https://github.com/janestreet/core_kernel/blob/5aa53f/lib/monad.ml#L44-L45

so the result of the 'include' is that Option ends up with a map
function as well.

Note that these are just the type signatures, not the actual
definitions.  The definitions themselves are found in the
implementation files:

   let map =
   match M.map with
   | `Define_using_bind -> map_via_bind
   | `Custom x -> x
   https://github.com/janestreet/core_kernel/blob/5aa53f/lib/monad.ml#L64-L67

However, the definition of 'map' in Core is split into several parts
that might make it tricky to understand if you don't have a lot of
experience reading OCaml code.  It might be easier to start by looking
at the more straightforward definition in the "batteries included"
library:

   let map f = function
   | None -> None
   | Some v -> Some (f v)
   https://github.com/ocaml-batteries-team/batteries-included/blob/47a416/src/batOption.ml#L36-L38