add CCResult.add_ctx{,f} for replacing stack traces

src/core/CCResult.ml

@@ -31,6 +31,22 @@ let fail_fprintf format =
     (fun out -> Format.pp_print_flush out (); fail (Buffer.contents buf))
     out format
 
+let add_ctx msg x = match x with
+  | Error e -> Error (e ^ "\ncontext:" ^ msg)
+  | Ok x -> Ok x
+
+let add_ctxf msg =
+  let buf = Buffer.create 64 in
+  let out = Format.formatter_of_buffer buf in
+  Format.kfprintf
+    (fun out e -> Format.pp_print_flush out (); add_ctx (Buffer.contents buf) e)
+    out msg
+
+(*$=
+   (Error "error\ncontext:message(number 42, foo: true)") \
+     (add_ctxf "message(number %d, foo: %B)" 42 true (Error "error"))
+*)
+
 let of_exn e =
   let msg = Printexc.to_string e in
   Error msg

src/core/CCResult.mli

@@ -42,6 +42,20 @@ val fail_fprintf : ('a, Format.formatter, unit, ('a, string) t) format4 -> 'a
 (** [fail_printf format] uses [format] to obtain an error message
     and then returns [Error msg] *)
 
+val add_ctx : string -> ('a, string) t -> ('a, string) t
+(** [add_ctx msg] leaves [Ok x] untouched, but transforms
+    [Error s] into [Error s'] where [s'] contains the additional
+    context given by [msg]
+    @since NEXT_RELEASE *)
+
+val add_ctxf : ('a, Format.formatter, unit, ('b, string) t -> ('b, string) t) format4 -> 'a
+(** [add_ctxf format_message] is similar to {!add_ctx} but with
+    {!Format} for printing the message (eagerly).
+    Example: {[
+      add_ctxf "message(number %d, foo: %B)" 42 true (Error "error)"
+    ]}
+    @since NEXT_RELEASE *)
+
 val map : ('a -> 'b) -> ('a, 'err) t -> ('b, 'err) t
 (** Map on success *)