/// <summary>
/// Retrieves the upcoming item without consuming it.
/// </summary>
/// <typeparam name="TItem">The item type of the stream.</typeparam>
/// <param name="stream">The <see cref="IStream{TItem}"/> to peek in.</param>
/// <returns>The next item in the stream.</returns>
public static TItem Peek <TItem>(this IPeekableStream <TItem> stream) => stream.LookAhead(0);
/// <summary>
/// Applies a parser to some input, consuming the parsed tokens on success.
/// </summary>
/// <typeparam name="TItem">The item type of the stream.</typeparam>
/// <typeparam name="TResult">The parsed value type.</typeparam>
/// <param name="parser">The parser to apply.</param>
/// <param name="stream">The stream to parse from.</param>
/// <returns>The result of parsing with <paramref name="parser"/> on <paramref name="stream"/>.</returns>
public static ParseResult <TResult> Parse <TItem, TResult>(this Parser <TItem, TResult> parser, IPeekableStream <TItem> stream)
{
var result = parser(stream, 0);
if (result.IsOk)
{
stream.Consume(result.Ok.Offset);
}
return(result);
}
/// <summary>
/// Peeks ahead a given amount of items without consuming them. With <paramref name="offset"/> set to 0
/// this is equivalent to <see cref="Peek"/>.
/// </summary>
/// <typeparam name="TItem">The item type of the stream.</typeparam>
/// <param name="stream">The <see cref="IPeekableStream{TItem}"/> to peek in.</param>
/// <param name="offset">The offset to look ahead.</param>
/// <returns>The peeked item.</returns>
public static TItem LookAhead <TItem>(this IPeekableStream <TItem> stream, int offset) => stream.TryLookAhead(offset, out var item)
? item
: throw new InvalidOperationException("The stream had no more items.");
