feat: add peek parser combinator

include/parsi/fn/peek.hpp

@@ -0,0 +1,26 @@
+#ifndef PARSI_FN_PEEK_HPP
+#define PARSI_FN_PEEK_HPP
+
+#include <type_traits>
+
+#include "parsi/base.hpp"
+
+namespace parsi::fn {
+
+/**
+ * A parser combinator that checks whether the given parser is valid or not
+ * without changing the stream cursor.
+ */
+template <is_parser F>
+struct Peek {
+    std::remove_cvref_t<F> parser;
+
+    [[nodiscard]] constexpr auto operator()(Stream stream) const noexcept -> Result
+    {
+        return Result{stream, parser(stream).is_valid()};
+    };
+};
+
+}  // namespace parsi::fn
+
+#endif  // PARSI_FN_PEEK_HPP

include/parsi/parsi.hpp

@@ -10,6 +10,7 @@
 #include "parsi/fn/expect.hpp"
 #include "parsi/fn/extract.hpp"
 #include "parsi/fn/optional.hpp"
+#include "parsi/fn/peek.hpp"
 #include "parsi/fn/repeated.hpp"
 #include "parsi/fn/sequence.hpp"
 #include "parsi/internal/optimizer.hpp"
@@ -196,6 +197,19 @@ template <is_parser F>
     return internal::optimize(fn::Optional<std::remove_cvref_t<F>>{std::forward<F>(parser)});
 }
 
+/**
+ * Creates a parser that only checks whether
+ * the given `parser` can parse the current
+ * stream, but does not move the stream forward.
+ * 
+ * @see fn::Peek
+ */
+template <is_parser F>
+[[nodiscard]] constexpr auto peek(F&& parser) noexcept
+{
+    return internal::optimize(fn::Peek<std::remove_cvref_t<F>>{std::forward<F>(parser)});
+}
+
 }  // namespace parsi
 
 #endif  // PARSI_PARSI_HPP

tests/parsers.cpp

@@ -129,6 +129,20 @@ TEST_CASE("extract")
                           [](std::string_view str) { return str == "not test"; })("test"));
 }
 
+TEST_CASE("peek")
+{
+    auto parser = pr::peek(pr::expect("x"));
+
+    CHECK(parser("x"));
+    CHECK(parser("x").stream().as_string_view() == "x");
+
+    CHECK(parser("xxa"));
+    CHECK(parser("xxa").stream().as_string_view() == "xxa");
+
+    CHECK(not parser("a"));
+    CHECK(parser("a").stream().as_string_view() == "a");
+}
+
 TEST_CASE("complex composition")
 {
     auto parser = pr::sequence(