Haskell <- 在 do 符号中

Question

众所周知，do 块中的 <- 只是 >>= 的语法糖，但 <- 在 Haskell 源代码或它只是作为语言语法一部分的句法构造，因此解析器只是用适当形式的 >>= 替换每个 <- 吗？

Answer 1

Do 表达式是语言的一部分。

来自 Haskell 2010 年语言报告：

3.14 Do Expression
lexp  →   do { stmts }            (do expression)
stmts →   stmt1 … stmtn exp [;]   (n ≥ 0)  
stmt  →   exp ;
      |   pat <- exp ;
      |   let decls ; 
      |   ;                       (empty statement)
A do expression provides a more conventional syntax for monadic programming. It allows an expression such as
 putStr "x: "    >>  
 getLine         >>= \l ->     
 return (words l)
to be written in a more traditional way as:
 do putStr "x: "  
    l <- getLine  
    return (words l)
As indicated by the translation of do, variables bound by let have fully polymorphic types while those defined by <- are lambda bound and are thus monomorphic.

请参阅 Haskell 语言报告的 Do Expression。

Haskell <- 在 do 符号中

Haskell <- in do notation

monads

haskell

functional-programming