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added README example that makes replacements in multiple files #230

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22 changes: 22 additions & 0 deletions README.md
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Expand Up @@ -220,6 +220,28 @@ script.Echo("a\nb\nc").FilterScan(func(line string, w io.Writer) {
// scanned line: "c"
```

Alternatively, we can use a `Filter` function that returns a `string`. For example, let's fix typos we've made in the CSV files in our working directory:
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I wonder if referencing CSV files is a red herring here. Readers might think this example is something to do with CSV parsing. Should we just talk about making arbitrary edits to all files matching a pattern?

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Sorry for disappearing for awhile! Yes I think that's better. I'll make that change.


```go
script.ListFiles("*.csv").FilterLine(func(file string) string {
search := "typ"
replace := "typo"
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Maybe we should use completely different search and replace strings, as this particular search and replace would not be idempotent: if you keep running it, "typo" will become "typoo", and so on.

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Good point! I'll change it to "helo" --> "hello".

s, err := File(file).String()
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Reading the whole file into a single string is a bit limiting—and unnecessarily so when we only need to read it line by line. What about using script's own Replace method, if that's possible here?

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That's a good point! I originally chose to read in the whole file like this so that I could use strings.Count. But I agree with your other comment that the counts are not essential to this example. I'll update my snippet to use Replace instead.

if err != nil {
return fmt.Sprintf("%s %s", file, err)
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Maybe it's better after all to use Filter, then, since that can return an error and stop the pipe if necessary; something doesn't feel right about just flattening it like this and carrying on. What do you think a sensible text-replacement program should do if it hits an error with some file? What would the equivalent sed command do?

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I created a test script to test what the equivalent sed would do:

$ bash test.sh
1.txt before sed: helo world!
3.txt before sed: helo there!
Running "sed -i 's/helo/hello/g' 1.txt 2.txt 3.txt"
sed: can't read 2.txt: No such file or directory
1.txt after sed: hello world!
3.txt after sed: hello there!

Even though sed doesn't behave this way, I do agree with you that it doesn't feel right to continue after encountering an error.

It seems like two approaches are:

  • Use Filter and stop the pipe at the first error
  • Use FilterLine and put any error messages on the pipe

With all of this in mind, which do you prefer? Or perhaps there is a third approach I'm not considering?

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Well, you're the customer: as the person who wants to replace text in multiple files, which behaviour would be more convenient or natural to you?

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Stopping at the first error seems more natural to me. I'll update my implementation accordingly.

}
count := strings.Count(s, search)
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It's nice to have a count of the number of replacements, but not essential—and the key to a good example is eliminating all non-essential detail. I don't deny that we probably would want a real script text-replacement program to do this, but examples like this are more effective when they're as brief and focused as possible, wouldn't you say?

_, err = Echo(strings.ReplaceAll(s, search, replace)).WriteFile(file)
if err != nil {
return fmt.Sprintf("%s %s", file, err)
}
return fmt.Sprintf("%s %d", file, count)
})
// Output:
// a.csv 3
// b.csv 0
```

And there's more. Much more. [Read the docs](https://pkg.go.dev/github.com/bitfield/script) for full details, and more examples.

# A realistic use case
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