Add Val_some,Val_none,Some_val, Is_none, Is_some, Tag_some (#9569)

Closes: #5154

Changes

@@ -30,6 +30,11 @@ Working version
   (KC Sivaramakrishnan, review by Stephen Dolan, Gabriel Scherer,
    and Xavier Leroy)
 
+- #9569: Add `Val_none`, `Some_val`, `Is_none`, `Is_some`, `caml_alloc_some`,
+  and `Tag_some`.
+  (Nicolás Ojeda Bär, review by Stephen Dolan, Gabriel Scherer, Mark Shinwell,
+  and Xavier Leroy)
+
 ### Code generation and optimizations:
 
 - #9441: Add RISC-V RV64G native-code backend.

manual/manual/cmds/intf-c.etex

@@ -654,6 +654,9 @@ containing \var{v} and \var{w} in fields 1 and 2.
 false otherwise
 \item "Is_block("\var{v}")" is true if value \var{v} is a pointer to a block,
 and false if it is an immediate integer.
+\item "Is_none("\var{v}")" is true if value \var{v} is "None".
+\item "Is_some("\var{v}")" is true if value \var{v} (assumed to be of option
+type) corresponds to the "Some" constructor.
 \end{itemize}
 
 \subsection{ss:c-int-ops}{Operations on integers}
@@ -668,6 +671,7 @@ truth value of the C integer \var{x}.
 \item "Bool_val("\var{v}")" returns 0 if \var{v} is the OCaml boolean
 "false", 1 if \var{v} is "true".
 \item "Val_true", "Val_false" represent the OCaml booleans "true" and "false".
+\item "Val_none" represents the OCaml value "None".
 \end{itemize}
 
 \subsection{ss:c-block-access}{Accessing blocks}
@@ -723,6 +727,8 @@ of a value \var{v} of any boxed type (record or concrete data type).
 \item "caml_field_unboxable("\var{v}")" calls either
 "caml_field_unboxed" or "caml_field_boxed" according to the default
 representation of unboxable types in the current version of OCaml.
+\item "Some_val("\var{v}")" returns the argument "\var{x}" of a value \var{v} of
+the form "Some("\var{x}")".
 \end{itemize}
 The expressions "Field("\var{v}", "\var{n}")",
 "Byte("\var{v}", "\var{n}")" and
@@ -791,6 +797,8 @@ any boxed type) whose field is the value \var{v}.
 \item "caml_alloc_unboxable("\var{v}")" calls either
 "caml_alloc_unboxed" or "caml_alloc_boxed" according to the default
 representation of unboxable types in the current version of OCaml.
+\item "caml_alloc_some("\var{v}")" allocates a block representing
+"Some("\var{v}")".
 \end{itemize}
 
 \subsubsection{sss:c-low-level-alloc}{Low-level interface}

runtime/alloc.c

@@ -272,3 +272,11 @@ CAMLprim value caml_update_dummy(value dummy, value newval)
   }
   return Val_unit;
 }
+
+CAMLexport value caml_alloc_some(value v)
+{
+  CAMLparam1(v);
+  value some = caml_alloc_small(1, 0);
+  Store_field(some, 0, v);
+  CAMLreturn(some);
+}

runtime/caml/alloc.h

@@ -49,6 +49,7 @@ CAMLextern value caml_alloc_sprintf(const char * format, ...)
   __attribute__ ((format (printf, 1, 2)))
 #endif
 ;
+CAMLextern value caml_alloc_some(value);
 
 CAMLextern value caml_alloc_with_profinfo (mlsize_t, tag_t, intnat);
 CAMLextern value caml_alloc_small_with_my_or_given_profinfo (

runtime/caml/mlvalues.h

@@ -373,6 +373,14 @@ CAMLextern header_t *caml_atom_table;
 #define Val_emptylist Val_int(0)
 #define Tag_cons 0
 
+/* Option constructors */
+
+#define Val_none Val_int(0)
+#define Some_val(v) Field(v, 0)
+#define Tag_some 0
+#define Is_none(v) ((v) == Val_None)
+#define Is_some(v) Is_block(v)
+
 /* The table of global identifiers */
 
 extern value caml_global_data;