意外的EOF与fmt.Scanner

huangapple
go评论109阅读模式
英文:

unexpected EOF with fmt.Scanner

问题

如果我想遍历一个字符串，可以这样做：

  1. package main
  3. import (
  4.    "fmt"
  5.    "strings"
  6. )
  8. func main() {
  9.    r := strings.NewReader("west north east")
  10.    for {
  11.       var s string
  12.       _, e := fmt.Fscan(r, &s)
  13.       fmt.Printf("%q %v\n", s, e)
  14.       if e != nil { break }
  15.    }
  16. }

结果：

  1. "west" <nil>
  2. "north" <nil>
  3. "east" <nil>
  4. "" EOF

我最近发现了fmt.Scanner[1]，所以我想尝试实现它。我想出了这个：

  1. package main
  3. import (
  4.    "fmt"
  5.    "strings"
  6. )
  8. type comma struct { tok string }
  10. func (c *comma) Scan(state fmt.ScanState, verb rune) error {
  11.    tok, err := state.Token(false, func(r rune) bool {
  12.       return r != ','
  13.    })
  14.    if err != nil {
  15.       return err
  16.    }
  17.    if _, _, err := state.ReadRune(); err != nil {
  18.       if len(tok) == 0 {
  19.          return err
  20.       }
  21.    }
  22.    c.tok = string(tok)
  23.    return nil
  24. }
  26. func main() {
  27.    r := strings.NewReader("west,north,east")
  28.    for {
  29.       var c comma
  30.       _, e := fmt.Fscan(r, &c)
  31.       fmt.Printf("%q %v\n", c.tok, e)
  32.       if e != nil { break }
  33.    }
  34. }

结果：

  1. "west" <nil>
  2. "north" <nil>
  3. "east" <nil>
  4. "" unexpected EOF

所以结果非常接近，但是让我困扰的是unexpected EOF。是否可能只使用自定义的fmt.Scanner获得常规的EOF？我在这里做错了什么，还是这是一个错误？

  1. https://golang.org/pkg/fmt#Scanner
英文:

If I want to scan through a string, I can do this:

  1. package main
  3. import (
  4.    &quot;fmt&quot;
  5.    &quot;strings&quot;
  6. )
  8. func main() {
  9.    r := strings.NewReader(&quot;west north east&quot;)
  10.    for {
  11.       var s string
  12.       _, e := fmt.Fscan(r, &amp;s)
  13.       fmt.Printf(&quot;%q %v\n&quot;, s, e)
  14.       if e != nil { break }
  15.    }
  16. }

Result:

  1. &quot;west&quot; &lt;nil&gt;
  2. &quot;north&quot; &lt;nil&gt;
  3. &quot;east&quot; &lt;nil&gt;
  4. &quot;&quot; EOF

I recently discovered fmt.Scanner [1], so I thought I would try to implement
it. I came up with this:

  1. package main
  3. import (
  4.    &quot;fmt&quot;
  5.    &quot;strings&quot;
  6. )
  8. type comma struct { tok string }
  10. func (c *comma) Scan(state fmt.ScanState, verb rune) error {
  11.    tok, err := state.Token(false, func(r rune) bool {
  12.       return r != &#39;,&#39;
  13.    })
  14.    if err != nil {
  15.       return err
  16.    }
  17.    if _, _, err := state.ReadRune(); err != nil {
  18.       if len(tok) == 0 {
  19.          return err
  20.       }
  21.    }
  22.    c.tok = string(tok)
  23.    return nil
  24. }
  26. func main() {
  27.    r := strings.NewReader(&quot;west,north,east&quot;)
  28.    for {
  29.       var c comma
  30.       _, e := fmt.Fscan(r, &amp;c)
  31.       fmt.Printf(&quot;%q %v\n&quot;, c.tok, e)
  32.       if e != nil { break }
  33.    }
  34. }

Result:

  1. &quot;west&quot; &lt;nil&gt;
  2. &quot;north&quot; &lt;nil&gt;
  3. &quot;east&quot; &lt;nil&gt;
  4. &quot;&quot; unexpected EOF

So the result is pretty close, but what bothers me is the unexpected EOF. Is
it possible to just get a regular EOF with a custom fmt.Scanner? Am I doing
something wrong here, or is this a bug?

  1. https://golang.org/pkg/fmt#Scanner

答案1

得分: 1

感谢 golang-nuts 列表上的 Ian Lance Taylor 的建议，他建议使用 panic 而不是 return 来处理错误。在 Go 代码中，Fscan 调用了一个名为 doScan 的函数，该函数又调用了一个名为 errorHandler 的函数 [1]。最后这个函数使用 recover 将任何 panic 转换为普通错误。这个程序产生了与我原始示例相同的输出：

  1. package main
  3. import (
  4.    "fmt"
  5.    "strings"
  6. )
  8. type comma struct { tok string }
  10. func (c *comma) Scan(state fmt.ScanState, verb rune) error {
  11.    tok, err := state.Token(false, func(r rune) bool {
  12.       return r != ','
  13.    })
  14.    if err != nil { return err }
  15.    if _, _, err := state.ReadRune(); err != nil {
  16.       if len(tok) == 0 {
  17.          panic(err)
  18.       }
  19.    }
  20.    c.tok = string(tok)
  21.    return nil
  22. }
  24. func main() {
  25.    r := strings.NewReader("west,north,east")
  26.    for {
  27.       var c comma
  28.       _, err := fmt.Fscan(r, &c)
  29.       fmt.Printf("%q %v\n", c.tok, err)
  30.       if err != nil { break }
  31.    }
  32. }
  1. https://github.com/golang/go/blob/go1.16.4/src/fmt/scan.go#L1056-L1067
英文:

Thanks to Ian Lance Taylor on the golang-nuts list, he suggested to panic
the error instead of return. In the Go code, Fscan calls a function
doScan, which in turn calls a function errorHandler [1]. This last function
uses recover to turn any panic into regular error. This program gives
idential output to my original example:

  1. package main
  3. import (
  4.    &quot;fmt&quot;
  5.    &quot;strings&quot;
  6. )
  8. type comma struct { tok string }
  10. func (c *comma) Scan(state fmt.ScanState, verb rune) error {
  11.    tok, err := state.Token(false, func(r rune) bool {
  12.       return r != &#39;,&#39;
  13.    })
  14.    if err != nil { return err }
  15.    if _, _, err := state.ReadRune(); err != nil {
  16.       if len(tok) == 0 {
  17.          panic(err)
  18.       }
  19.    }
  20.    c.tok = string(tok)
  21.    return nil
  22. }
  24. func main() {
  25.    r := strings.NewReader(&quot;west,north,east&quot;)
  26.    for {
  27.       var c comma
  28.       _, err := fmt.Fscan(r, &amp;c)
  29.       fmt.Printf(&quot;%q %v\n&quot;, c.tok, err)
  30.       if err != nil { break }
  31.    }
  32. }
  1. https://github.com/golang/go/blob/go1.16.4/src/fmt/scan.go#L1056-L1067

huangapple
  • 本文由 发表于 2021年5月27日 08:27:32
  • 转载请务必保留本文链接：https://go.coder-hub.com/67714217.html
  • eof
  • go
  • string
  • token
匿名

发表评论

匿名网友

:?: :razz: :sad: :evil: :!: :smile: :oops: :grin: :eek: :shock: :???: :cool: :lol: :mad: :twisted: :roll: :wink: :idea: :arrow: :neutral: :cry: :mrgreen:

确定