Recognizing Covariance Issues in Your Code

My last three posts have covered the nuts and bolts of the little-used in and out keywords:

The examples have all been small, and a little contrived, so you may not yet recognize when you need to fetch this tool from your toolbelt. Today, I’ll cover a real covariance problem I ran into while developing Parsley, an open-source text parsing library.

The goal of parsing is to convert plain text into a data structure that can be more easily traversed and reasoned about. You want to describe the patterns of what valid input looks like, as well as describe what objects should be created when those patterns are found.

In today’s example, we’ll be parsing JSON text into the corresponding arrays and dictionaries that JSON describes. Parsley’s integration tests include a JSON parser. Without the out keyword, this sample parser would fail to compile with some pretty suspicious error messages.

The most important types defined in Parsley are Parser<T> and Reply<T>:

public interface Parser<out T>
    Reply<T> Parse(Lexer tokens);

public interface Reply<out T>
    T Value { get; }
    Lexer UnparsedTokens { get; }
    bool Success { get; }
    ErrorMessageList ErrorMessages { get; }

A Lexer (aka tokenizer) inspects the input plain text and converts it to a series of tokens. If we’re parsing JSON, tokens would be things like a constant, a keyword, or a puctuation symbol like [ or {. A Parser is any object which can consume some tokens and produce a Reply. A Reply contains the parsed object (if the input was valid), the remaining unconsumed tokens, and possibly some error messages.

Parsley lets you take small parsers that do part of the effort, and glue them together into larger parsers that do the full parsing effort. Like regexes, the glue involves repetition (“zero or more of that pattern”), alternation (“any one of these possible patterns”), and sequencing (“this pattern, followed by that pattern”).


For repetition, we have helper functions that take in a Parser<T> and output a new Parser<IEnumerable<T>>. The new parser will repeatedly recognize whether the pattern appears again, consuming it until the pattern is no longer recognized. The overall Reply will contain an object for every occurrence of the pattern:

public static Parser<IEnumerable<T>> ZeroOrMore<T>(Parser<T> item) { ... }
public static Parser<IEnumerable<T>> OneOrMore<T>(Parser<T> item) { ... }

Parser<char> digit = ...;
Parser<IEnumerable<char>> integer = ZeroOrMore(digit);


For alternation, the Choice helper function takes in several Parser<T> and returns a new Parser<T> which recognizes when any one of those patterns appears next in the input text:

public static Parser<T> Choice<T>(params Parser<T>[] parsers) { ... }

//The result of a successful parsing of JSON text
//is the result of one of these other simpler patterns,
//whichever one is encountered first:

Json.Rule =
    Choice(True, False, Null, Number, Quotation, Dictionary, Array);


For sequencing, I redefined the from and select LINQ keywords to provide the sequencing operation, gluing together small Parsers into a larger Parser.

In part of the JSON parser, we need to recognize the pattern of a single key/value pair for a dictionary. Such a pair is a quoted string, followed by a ‘:’, followed by some other JSON value. The result of the Pair rule is an object containing the relevant parts of that sequence:

Pair.Rule =
    from key in Quotation
    from colon in Token(":")
    from value in Json
    select new KeyValuePair<string, object>(key, value);

The result of a query like this is yet another Parser. Each from clause consumes a little more input, and the select clause describes the combined result of those intermediate steps.

The Problem

At one point, before the Parser and Reply types used the out keyword, I ran into some surprising compiler errors. I wanted to write the following:

GrammarRule<object[]> Array;
GrammarRule<Dictionary<string, object>> Dictionary;


Array.Rule =
    from items in Between(Token("["), ZeroOrMore(Json, Token(",")), Token("]"))
    select items.ToArray();
Dictionary.Rule =
    from pairs in Between(Token("{"), ZeroOrMore(Pair, Token(",")), Token("}"))
    select ToDictionary(pairs);

Just to satisfy the compiler, though, I had to write this instead:

//Rule declarations had to become less precise:

GrammarRule<object> Array;
GrammarRule<object> Dictionary;


//Select clauses needed explicit upcasting:

Array.Rule =
    from items in Between(Token("["), ZeroOrMore(Json, Token(",")), Token("]"))
    select (object) items.ToArray();
Dictionary.Rule =
    from pairs in Between(Token("{"), ZeroOrMore(Pair, Token(",")), Token("}"))
    select (object) ToDictionary(pairs);

In order to pass Array and Dictionary to the Choice helper method, they needed to have the same type as each other. In order to satisfy the compiler after that, I was having to also include explicit upcasts in the select clauses. That is just weird! Have you ever had to explicitly upcast, just to satisfy the compiler? As if an array or dictionary might not be an object!

The Solution

When the compiler surprises you, the most direct path to getting a successful compile isn’t necessarily the right answer. First, try to understand what the compiler is trying to tell you. Ask yourself, “What is it about my code that the compiler is getting tripped up on? Why isn’t it making the decision I expected?”

Taking a closer look, these queries result in values of type Parser<object[]> and Parser<Dictionary<string, object>>, respectively, and we’re trying to assign them each to a .Rule property of type Parser<object>.

We want to treat any Parser<Child> as a subtype of any Parser<Parent>, whenever Child is a subtype of Parent. When we look at the Parser<T> and Reply<T> definitions, we see that the Ts always appear in the return types, never in the input types. The out keyword is therefore applicable here, with the desired results:

  1. Using Choice no longer has the suspicious side effect of requiring less-descriptive types for each rule we happen to pass to it.
  2. The compiler no longer needs us to include suspicious explicit upcasts in our select clauses.