形式的模式匹配:Option {..}<- [英] pattern matching of the form: Option{..} <-

问题描述

这种模式匹配的形式是什么:Option{..} <- ...，例如如此处所用:

What is this form of pattern matching called:Option{..} <- ..., e.g. as it is used here:

data Option = Option { cabal :: Maybe String , noStylish :: Bool }
...
main = do
  Option{..} <- cmdArgs defOption
  cabp <- case cabal of
    Nothing -> do
    ...

似乎重新定义了cabal和nostylish.模式匹配cabal之前的类型为Option -> Maybe String，但是之后为Maybe String.

It seems to redefine cabal and nostylish. Before the pattern match cabal has type Option -> Maybe String but after it has type Maybe String.

此示例来自最近上传的软件包cabal2ghci.

This example comes from the recently uploaded package cabal2ghci.

推荐答案

这是GHC语法扩展，称为

This is a GHC syntactic extension called record wildcards. Quoting documentation:

记录通配符语法允许在记录模式中使用".."，其中每个被忽略的字段f被模式f = f代替.

Record wildcard syntax permits a ".." in a record pattern, where each elided field f is replaced by the pattern f = f.

所以这段代码等同于

Option { cabal = cabal, noStylish = noStylish } <- cmdArgs defOption

为记录类型中的每个字段有效地将名称x绑定到名为x的记录字段的值.

effectively binding name x to the value of record field named x for every field in the record type.

<-部分与此处无关，您也可以编写

<- part is irrelevant here, you can as well write

let Option { .. } = some expression

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