On Thu, May 27, 2021 at 2:25 PM Steven Penny <srp...@gmail.com> wrote:
>
> Ian, that doesnt make sense. Then how will the scanner know that the input is
> empty? EOF is the only graceful error I know for this case, its the only
> sentinel that I am aware of, to differentiate a "real error" from "input is
> empty".
You're right, I took a look at the regular scanners, and they call
panic(io.EOF) when they hit EOF. Try changing your Scan method to do
that and see if that works. If it does, we ought to clarify that in
the documentation of fmt.Scanner.
Ian
> On Thursday, May 27, 2021 at 4:10:06 PM UTC-5 Ian Lance Taylor wrote:
>>
>> On Thu, May 27, 2021 at 6:41 AM Steven Penny <srp...@gmail.com> wrote:
>> >
>> > If I want to scan through a string, I can do this:
>> >
>> > ~~~go
>> > package main
>> >
>> > import (
>> > "fmt"
>> > "strings"
>> > )
>> >
>> > func main() {
>> > r := strings.NewReader("west north east")
>> > for {
>> > var s string
>> > _, e := fmt.Fscan(r, &s)
>> > fmt.Printf("%q %v\n", s, e)
>> > if e != nil { break }
>> > }
>> > }
>> > ~~~
>> >
>> > Result:
>> >
>> > ~~~
>> > "west" <nil>
>> > "north" <nil>
>> > "east" <nil>
>> > "" EOF
>> > ~~~
>> >
>> > I recently discovered `fmt.Scanner` [1], so I thought I would try to
>> > implement it. I came up with this:
>> >
>> > ~~~go
>> > package main
>> >
>> > import (
>> > "fmt"
>> > "strings"
>> > )
>> >
>> > type comma struct { tok string }
>> >
>> > func (c *comma) Scan(s fmt.ScanState, r rune) error {
>> > tok, err := s.Token(false, func(r rune) bool {
>> > return r != ','
>> > })
>> > if err != nil {
>> > return err
>> > }
>> > c.tok = string(tok)
>> > if _, _, err := s.ReadRune(); err != nil {
>> > return err
>> > }
>> > return nil
>> > }
>> >
>> > func main() {
>> > r := strings.NewReader("west,north,east")
>> > for {
>> > var c comma
>> > _, e := fmt.Fscan(r, &c)
>> > fmt.Printf("%q %v\n", c.tok, e)
>> > if e != nil { break }
>> > }
>> > }
>> > ~~~
>> >
>> > Result:
>> >
>> > ~~~
>> > "west" <nil>
>> > "north" <nil>
>> > "east" unexpected EOF
>> > ~~~
>> >
>> > So the result is pretty similar, but what bothers me is the `unexpected
>> > EOF`. It seems it is due to this code:
>> >
>> > https://github.com/golang/go/blob/3075ffc93e962792ddf43b2a528ef19b1577ffb7/src/fmt/scan.go#L956-L966
>> >
>> > It seems like `EOF` should be valid in this case, or perhaps I dont
>> > understand the reasoning for it to be unexpected.
>>
>> If you want to use fmt.Scanner, the Scan method of the types you are
>> using should not return an acceptable io.EOF. That doesn't make sense
>> as an error from a Scan method. A Scan method that encounters io.EOF
>> at the end of valid input should store the valid result and return
>> nil.
>>
>> Ian
>
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