Haskell:如何评估像“1+2"这样的字符串 [英] Haskell: how to evaluate a String like "1+2"

问题描述

实际上我有一些像 "x + y" 这样的公式，它是一个 String.我设法将 x/y 变量替换为特定值，例如 "1.2"，它仍然是 String 类型.现在我有像 "1 + 2" 这样的表达式.

Actually I have some formula like "x + y", which is a String. I managed to replace the x/y variable with specific values like "1.2", which is still String type. Now I have expression like "1 + 2".

所以问题是如何对字符串类型的表达式求值并得到结果.

So the problem is how to evaluate a expression of a string type and get the result.

ps:我想像 read 这样的东西，它可以直接转换整个字符串表达式，而不是逐个处理运算符(+/- 等).那可能吗?

ps: I wanna sth like read, that can directly convert the whole string expression instead of handling the operator (+/-,etc) case by case. Is that possible?

推荐答案

你的问题留下了很大的解释空间.我猜你不习惯构建一个完整的词法分析、解析、类型检查和评估管道.长答案将涉及您定义要评估的语言(只是带有+"的整数，也许所有带有+"、-"、*"、/"甚至更大的语言的有理数?)并执行每个该语言的上述步骤.

Your question leaves a lot of room for interpretation. I'm taking a guess you aren't accustom to building a whole pipeline of lexing, parsing, maybe type checking, and evaluating. The long answer would involve you defining what language you wish to evaluate (Just integers with '+', perhaps all rationals with '+', '-' '*', '/', or even a larger language?) and perform each of the above steps for that language.

简短的回答是:要评估 Haskell 表达式，其中包括您可能正在谈论的基本数学运算符，只需使用提示"包:

The short answer is: to evaluate Haskell expressions, which includes the basic math operators you're probably talking about, just use the "hint" package:

$ cabal install hint
...
$ ghci
> import Language.Haskell.Interpreter
> runInterpreter $ setImports ["Prelude"] >> eval "3 + 5"
Right "8"

耶！

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