swift-parsing-dump

[randomeizer]

I put together a little tool to help with debugging complex parsers called swift-parsing-dump.

Basically, it lets you do a customDump on the input and output of either parse or print calls.

For example, you have an Email type and a parser for it:

struct EmailAddress: Equatable {
  let username: String
  let server: String
}
let emailParser = Parse(EmailAddress.init(username:server:)) {
  PrefixThrough("@").map(.string)
  "@"
  Rest().map(.string)
}

let email = try emailParser.parse("[email protected]")

Unfortunately, this results in an error:

error: unexpected input
 --> input:1:5
1 | [email protected]
  |     ^ expected "@"

"But why?" you ask. There is an "@" right there! Let's do some dumping:

let emailParser = Parse(EmailAddress.init(username:server:)) {
  PrefixThrough("@").map(.string).dump("username")
  "@".dump("@")
  Rest().map(.string).dump("server")
}

let email = try emailParser.parse("[email protected]")

Now, when I run it (in DEBUG), I get this in the console:

username: parse(Substring) -> String
input:
- "[email protected]"
+ "bar.com"
output:
"foo@"
@: parse(Substring) -> ()
input:
bar.com
error:
ParsingError.failed(
  "expected \"@\"",
  ParsingError.Context(
    debugDescription: "unexpected input",
    originalInput: "bar.com",
    remainingInput: "bar.com",
    underlyingError: nil
  )
)

Oh, right, PrefixThrough consumes the delimiter character. I just need to switch to PrefixUpTo("@") instead:

let emailParser = Parse(EmailAddress.init(username:server:)) {
  PrefixUpTo("@").map(.string).dump("username")
  "@".dump("@")
  Rest().map(.string).dump("server")
}

Now I run it, no errors, and I get this in the console:

username: parse(Substring) -> String
input:
- "[email protected]"
+ "@bar.com"
output:
"foo"
@: parse(Substring) -> ()
input:
- "@bar.com"
+ "bar.com"
output:
()
server: parse(Substring) -> String
input:
- "bar.com"
+ ""
output:
"bar.com"

Sweet.

Obviously a simple example, but parsers can get quite nested, and busting out specific-case unit tests can take time. It's also limited when working with more complex parsers like OneOf or Many, since it's running multiple parsers under the hood.

Anyway, hopefully that's useful to someone else out there!