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Parser.Not.cs
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Parser.Not.cs
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using System;
using System.Collections.Generic;
using System.Collections.Immutable;
using System.Linq;
namespace Pidgin
{
public static partial class Parser
{
/// <summary>
/// Creates a parser which succeeds only if the given parser fails.
/// The resulting parser does not perform any backtracking; it consumes the same amount of input as the supplied parser.
/// Combine this function with <see cref="Parser.Try{TToken, T}(Parser{TToken, T})"/> if this behaviour is undesirable.
/// </summary>
/// <param name="parser">The parser that is expected to fail</param>
/// <returns>A parser which succeeds only if the given parser fails.</returns>
public static Parser<TToken, Unit> Not<TToken, T>(Parser<TToken, T> parser)
{
if (parser == null)
{
throw new ArgumentNullException(nameof(parser));
}
return new NegatedParser<TToken, T>(parser);
}
private sealed class NegatedParser<TToken, T> : Parser<TToken, Unit>
{
private readonly Parser<TToken, T> _parser;
public NegatedParser(Parser<TToken, T> parser)
{
_parser = parser;
}
internal sealed override InternalResult<Unit> Parse(ref ParseState<TToken> state)
{
var startingPosition = state.SourcePos;
var token = state.HasCurrent ? Maybe.Just(state.Current) : Maybe.Nothing<TToken>();
state.BeginExpectedTran();
var result = _parser.Parse(ref state);
state.EndExpectedTran(false);
if (result.Success)
{
state.Error = new InternalError<TToken>(
token,
false,
startingPosition,
null
);
return InternalResult.Failure<Unit>(result.ConsumedInput);
}
return InternalResult.Success(
Unit.Value,
result.ConsumedInput
);
}
}
}
}