避免显式记录类型

Question

假设我有以下操作理想化流的函数：

fun Stream s = { pos = 0, row = 1, col = 0, str = s }
fun len { str, pos = _, row = _, col = _ } = String.size str

fun poke off { str, pos, row: int, col: int } =
  let val n = pos + off in
    if n >= 0 andalso n <= (String.size str) then SOME (String.sub(str, n)) else NONE
  end

这 works/compiles，但不幸的是，我的函数定义中充满了我不关心的信息。 row/col 被忽略 poke 和 len。但是，虽然通配符可以与 len 一起使用，但不能与 poke 一起使用。有没有一种方法可以重构这些函数，以便不需要输入那么明确的输入，同时仍然能够模式 match/destructure?

Answer 1

如果你给你的类型起个名字（比如stream），你可以更简单地引用它：

type stream = { pos : int, row : int, col : int, str : string }

fun Stream s = { pos = 0, row = 1, col = 0, str = s }

fun len ({ str, ... } : stream) = String.size str

fun poke off ({ str, pos, ... } : stream) =
  let val n = pos + off in
    if n >= 0 andalso n <= String.size str
    then SOME (String.sub (str, n))
    else NONE
  end

或者，或多或少等同于：

datatype stream = STREAM of { pos : int, row : int, col : int, str : string }

fun Stream s = STREAM { pos = 0, row = 1, col = 0, str = s }

fun len (STREAM { str, ... }) = String.size str

fun poke off (STREAM { str, pos, ... }) =
  let val n = pos + off in
    if n >= 0 andalso n <= String.size str
    then SOME (String.sub (str, n))
    else NONE
  end

避免显式记录类型

Avoiding explicit record typing

record

sml

smlnj