Parser Errors

Monday February 14, 2022

We are excited to release 0.7.0 of our swift-parsing library that brings delightful and informative error messaging to parser failures. This is a huge change to the library, and unfortunately is a breaking change, but continue reading to see how to migrate existing parsers and learn a new trick using an experimental Swift compiler feature.

What’s different?

The most fundamental change is that the Parser protocol’s single method requirement is now a throwing function rather an optional-returning function:

 public protocol Parser {
   associatedtype Input
   associatedtype Output
-  func parse(_ input: inout Input) -> Output?
+  func parse(_ input: inout Input) throws -> Output
 }

From the beginning of swift-parsing we leaned on optionals as a very simple way to denote failure when parsing. That works well enough for simple parsers, but as parsers become more and more complex, a proper description and context is needed to understand where a parser failed and why.

For example, if we want to parse a CSV formatted string into an array of User struct values:

struct User {
  var id: Int
  var name: String
  var admin: Bool
}

let input = """
1,Blob,true
2,Blob Jr.,false
3,Blob Sr.,tru
"""

We can do so with a few seemingly-simple parsers:

let user = Parse(User.init) {
  Int.parser()
  ","
  Prefix { $0 != "," }.map(String.init)
  ","
  Bool.parser()
}

let users = Many {
  user
} separator: {
  "\n"
} terminator: {
  End()
}

But, if we run our parser on the input value above we will find we only get nil instead of an array of users:

let output = users.parser(input)  // nil

The reason for this is because we actually have a typo for the boolean in the last column of the last row of the CSV text:

let input = """
1,Blob,true
2,Blob Jr.,false
3,Blob Sr.,tru
"""

That typo causes parsing to fail, but we have no information of what went wrong since the parser just returns nil. This can be really frustrating for large inputs where it can be difficult to find where the error is, especially when the error is literally the last character of the input.

By making the parse method of the Parser protocol throwing we can contextualize the error message in a much better way. For example, calling the throwing parse method on the above malformed input:

let output = try users.parse(input)

An error is now thrown, and it shows exactly what went wrong, including pointing to the exact line and character where the error occured:

caught error: "error: multiple failures occurred

error: unexpected input
 --> input:3:11
3 | 3,Blob Jr,tru
  |           ^ expected "true" or "false"

error: unexpected input
 --> input:2:16
2 | 2,Blob Sr,false
  |                ^ expected end of input"

There are two errors printed because technically two things went wrong:

  • First and foremost the boolean parser failed to parse “tru”.

  • Second, because the boolean parser failed it caused the Many parser to only consume the first 2 lines of the input text. This left additional input to be consumed, but we told our Many parser that it should consume the full input by using End() for its terminator.

This can be incredibly handy for tracking down logical problems in your parsers or figuring out what is wrong with the input.

Migrating from 0.6.0

Unfortunately changing the Parser protocol’s requirement to be throwing is a breaking change, and we’re not sure there is a way to maintain backwards compatibility. But fortunately there are a few small things you can do to bring your code up-to-date.

By far the most common use of parsers that will need to be migrated is when calling the parse method. To recapture that behavior you only need to try? the parsing:

- let output = myParser.parse(&input)
+ let output = try? myParser.parse(&input)

However, if you want to get access to the error message you can open a do block to try the parser, and when catching an error you can print it:

do {
  output = try myParser.parser(&input)
} catch {
  print(error)
}

A less common, but still important, use of parsers that will need to be migrated are custom conformances to the parser protocol.

If you have a custom parser type, you will need to update its parse method to be throwing. Depending on the parser, this can give you an opportunity to clean up code. For instance, Void parsers no longer need to explicitly and awkwardly return an optional void value:

 struct End: Parser {
-  func parse(_ input: inout Substring) -> Void? {
+  func parse(_ input: inout Substring) throws {
     guard input.isEmpty else {
-      return nil
+      throw …
     }
-    return ()
   }
 }

And parsers that call out to other parsers under the hood can prefer try over optional wrangling:

 struct Map<Upstream: Parser, NewOutput>: Parser {
   let upstream: Upstream
   let transform: (Upstream.Output) -> NewOutput

-  func parse(_ input: inout Upstream.Input) -> NewOutput?
-    guard let output = self.upstream.parse(&input)
-    else { return nil }
+  func parse(_ input: inout Upstream.Input) throws -> NewOutput {
+    self.transform(try self.upstream.parse(&input))
   }
 }

Notably, types can satisfy throwing protocol conformances without throwing themselves, which means if your parser cannot fail, you can drop the throws and try for parsers that can’t fail.

public struct Rest: Parser {
  public func parse(_ input: inout Substring) -> Input {
    let output = input
    input.removeFirst(input.count)
    return output
  }
}

// No need to `try`!
Rest().parse("Hello!")  // "Hello!"

This means you can even use the new .replaceError(with:) parser operator throughout your parsers to get compile-time guarantees that parsing can’t fail.

// No need to `try`!
Int.parser().replaceError(with: 0).parse("!!!")  // 0

All of the parsers and operators that ship with the library throw a concrete error whose type is not currently made public. It may be publicized in a future release, but for now we want more flexibility for changing the type without breaking backwards compatibility.

However, you can still throw your own custom error messages and it will be reformatted and contextualized. For example, suppose we wanted a parser that only parsed the digits 0-9 from the beginning of a string and transformed it into an integer. This is subtly different from Int.parser() which allows for negative numbers.

Constructing a Digits parser is easy enough, and we can introduce a custom struct error for customizing the message displayed:

struct DigitsError: Error {
  let message = "Expected a prefix of digits 0-9"
}

struct Digits: Parser {
  func parse(_ input: inout Substring) throws -> Int {
    let digits = input.prefix { $0 >= "0" && $0 <= "9" }
    guard let output = Int(digits)
    else {
      throw DigitsError()
    }
    input.removeFirst(digits.count)
    return output
  }
}

If we swap out the Int.parser for a Digits parser in user:

 let user = Parse(User.init) {
-  Int.parser()
+  Digits()
   ","
   Prefix { $0 != "," }.map(String.init)
   ","
   Bool.parser()
 }

And we introduce an incorrect value into the input:

  let input = """
  1,Blob,true
- 2,Blob Jr.,false
+ -2,Blob Jr.,false
  3,Blob Sr.,true
  """

Then when running the parser we get a nice error message that shows exactly what went wrong:

error: DigitsError(message: "Expected a prefix of digits 0-9")
 --> input:2:1
2 | -2,Blob Sr,false
  | ^

@rethrows

In this release, swift-parsing has adopted an experimental compiler feature known as rethrowing protocol conformances (Swift forums post).

As is well-known, non-throwing functions can satisfy throwing protocol requirements. This can be incredibly powerful, allowing protocol conformances to more correctly describe their behavior. However, rethrowing functions cannot satisfy throwing protocol requirements. Take for instance the conformance of Parsers.Map to the Parser protocol:

struct Map<Upstream: Parser, NewOutput>: Parser {
  let upstream: Upstream
  let transform: (Upstream.Output) -> NewOutput

  func parse(_ input: inout Upstream.Input) throws -> NewOutput {
    self.transform(try self.upstream.parse(&input))
  }
}

The parse method above is marked as throws even though it technically does not itself throw any errors. It only throws when upstream throws, and so if upstream does not throw then we would love if Parsers.Map could be inferred to not throw. That is exactly what rethrows allows us to do, but if we use it here:

func parse(_ input: inout Upstream.Input) rethrows -> NewOutput {
  …
}

‘rethrows’ function must take a throwing function argument

We get an error that tells us we cannot use rethrows in this way. This means if you .map on a parser that does not fail you will get back a parser that fails, even though the compiler should statically know it’s impossible to fail.

For example, the Whitespace parser does not fail, and if we wanted to .map on it to count how much whitespace an input begins with we would unnecessarily turn it into a failing parser:

let whitespaceCount = Whitespace().map(\.count)
try whitespaceCount.parse("   Hello".utf8)  // 3
// Must use `try`, even though the parser cannot fail 😫

This is a problem even in the Swift standard library. For example, the IteratorProtocol is designed like so:

protocol IteratorProtocol {
  associatedtype Element

  mutating func next() -> Element?
}

This represents a stream of elements that can be pulled by invoking next(), and once the stream is empty it can return nil to signify completion.

However, this means that pulling elements from a stream is never allowed to signify failure by throwing an error. This is not very realistic as many streams can fail. Even something as simple as reading lines from stdin can fail, and right now we would have to throw away any failures and just return nil:

struct ReadLineIterator: IteratorProtocol {
  mutating func next() -> String? {
    try? readline()
  }
}

The IteratorProtocol was designed in this way because the compiler lacked rethrowing protocol conformances, just as we saw above with Parsers.Map. Ideally we could define IteratorProtocol with a throwing next() method like this:

protocol IteratorProtocol {
  associatedtype Element

  mutating func next() throws -> Element?
}

And then conformances could choose to be throwing, non-throwing and re-throwing depending on the situation. But, as we saw with Parsers.Map above, that is just not possible in today’s Swift.

This problem is exactly what Swift’s experimental @rethrows feature aims to solve. By marking our Parser protocol definition as @rethrows:

@rethrows public protocol Parser {
  associatedtype Input
  associatedtype Output

  func parse(_ input: inout Input) throws -> Output
}

We instantly get the ability for conformances of Parser to rethrow their errors rather than being throwing themselves:

 struct Map<Upstream: Parser, NewOutput>: Parser {
   let upstream: Upstream
   let transform: (Upstream.Output) -> NewOutput

-  func parse(_ input: inout Upstream.Input) throws -> NewOutput {
+  func parse(_ input: inout Upstream.Input) rethrows -> NewOutput {
     self.transform(try self.upstream.parse(&input))
   }
 }

And now mapping on a whitespace parser produces a new producer that is known by Swift to not fail:

let whitespaceCount = Whitespace().map(\.count)
whitespaceCount.parse("   Hello".utf8) // 3
// No need to `try` since Swift knows the parser cannot fail! 😄

This allows parsers to prove more about what it does and does not do to the compiler, and can make parsers (and other protocols) give strong guarantees.

Although the @rethrows attribute is completely experimental and still has not passed the muster of Swift evolution, it is currently being used in Swift’s new concurrency APIs, where AsyncIteratorProtocol and AsyncSequence have been designed so as not to suffer the same shortcomings of IteratorProtocol and Sequence:

@rethrows public protocol AsyncIteratorProtocol {
  …
}

@rethrows public protocol AsyncSequence {
  …
}

Start using it today!

Version 0.7.0 is available today, so once you are ready to migrate your parsers to throwing be sure to let us know how it is working out for you!

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