从类型中选择的结构成员，它是不可见的，不会被选择

Question

我有一个使用 Unix 库作为时间函数的函数：

let rfc822 (t: Unix.tm) : string =
    Printf.sprintf "%s, %s %s %d %s:%s:%s %s"
    (List.nth short_days t.tm_wday)
    (padInt t.tm_yday 2 "0")
    (List.nth short_month t.tm_mon)
    (t.tm_year + 1900)
    (padInt t.tm_hour 2 "0")
    (padInt t.tm_min 2 "0")
    (padInt t.tm_sec 2 "0")
    "GMT"

我收到此警告：

ocamlbuild -libs unix,str -Is recore/src,ostd/src,owebl/src app.native
+ /usr/bin/ocamlc -c -I recore/src -I ostd/src -I owebl/src -o recore/src/time.cmo recore/src/time.ml
File "recore/src/time.ml", line 45, characters 27-34:
Warning 40: tm_wday was selected from type Unix.tm.
It is not visible in the current scope, and will not 
be selected if the type becomes unknown.
File "recore/src/time.ml", line 46, characters 14-21:
Warning 40: tm_yday was selected from type Unix.tm.
It is not visible in the current scope, and will not 
be selected if the type becomes unknown.
File "recore/src/time.ml", line 46, characters 4-28:
Error: This expression has type 'a -> string
   but an expression was expected of type string
Command exited with code 2.
Compilation unsuccessful after building 13 targets (12 cached) in 00:00:00.
Makefile:8: recipe for target 'old' failed
make: *** [old] Error 10

如何处理这个警告？如果可能的话，我宁愿避免 opening Unix 模块。

(实际编译错误请忽略)

Answer 1

你可以写t.Unix.tm_yday

$ ocaml
        OCaml version 4.02.1

# let f (t: Unix.tm) = t.tm_yday;;
Warning 40: tm_yday was selected from type Unix.tm.
It is not visible in the current scope, and will not
be selected if the type becomes unknown.
val f : Unix.tm -> int = <fun>

# let f (t: Unix.tm) = t.Unix.tm_yday;;
val f : Unix.tm -> int = <fun>

更新

要在文档中找到它，您需要查找 field 的定义：

field   ::= [ module-path . ]  field-name

字段名称可以在字段名称本身之前包含一个模块名称（或一系列模块名称，对于嵌套模块）。

更新 2

本地打开模块也有两种语法。对于这个微小的功能，它们看起来有点矫枉过正，但对于更复杂的功能来说可能更整洁。模块的符号在整个子表达式中直接可用。

$ ocaml
        OCaml version 4.02.1

# let f t = Unix.(t.tm_yday);;
val f : Unix.tm -> int = <fun>

# let f t = let open Unix in t.tm_yday;;
val f : Unix.tm -> int = <fun>

这些在 Local opens 中记录为语言扩展。

从类型中选择的结构成员，它是不可见的，不会被选择

Struct member selected from type, it is not visible and will not be selected

unix

ocaml