假设我具有以下用于在理想化流上进行操作的功能:
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
这可以工作/编译,但是不幸的是我不得不用我不关心的信息来乱放我的函数定义。 row
/ col
被忽略poke
和len
。但是,尽管通配符可以与len
一起使用,但不能与poke
一起使用。有没有一种方法可以重组这些功能,以便在不影响显式键入的情况下仍然能够进行模式匹配/解构?
答案 0 :(得分:1)
如果为您的类型命名(例如stream
),则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,或多或少等效:
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