使用ml-lex构建一个词法分析器 [英] building a lexical analyser using ml-lex

问题描述

我需要创建与标准输入流绑定的lexer的新实例.
但是，当我输入

I need to create a new instance of a lexer tied to the standard input stream.
However, when I type in

val lexer = makeLexer( fn n => inputLine( stdIn ) );

我收到一个我不理解的错误:

I get an error that I don't understand:

stdIn:1.5-11.13 Error: operator and operand don't agree [tycon mismatch]
  operator domain: int -> string
  operand:         int -> string option
  in expression:

(makeLexer是我的源代码中存在的函数名称)

(makeLexer is a function name present in my source code)

推荐答案

inputLine 返回一个string option，我猜是一个string.

inputLine returns a string option, and my guess is a string is expected.

您想要做的就是让makeLexer取一个string option，就像这样:

What you want to do is either have makeLexer take a string option, like so:

fun makeLexer  NONE    = <whatever you want to do when stream is empty>
  | makeLexer (SOME s) = <the normal body makeLexer, working on the string s>

或将您的行更改为:

val lexer = makeLexer( fn n => valOf ( inputLine( stdIn ) ) );

valOf 提取选项类型并将其解压缩

valOf takes an option type and unpacks it.

请注意，由于当流为空时inputLine返回NONE，使用第一种方法而不是第二种方法可能是一个更好的主意.

Note that, since inputLine returns NONE when the stream is empty, it's probably a better idea to use the first approach, rather than the second.

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