1 files changed, 61 insertions, 0 deletions
diff --git a/src/nibble/parse/mod.rs b/src/nibble/parse/mod.rs
new file mode 100644
index 0000000..13c4c0e
--- /dev/null
+++ b/src/nibble/parse/mod.rs
@@ -0,0 +1,61 @@
+pub mod iter;
+
+use self::iter::Peek;
+use self::iter::PeekExt;
+
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub enum ParseError<E> {
+    EndOfFile,
+    InvalidCharacter(char),
+    Other(E),
+}
+
+impl<U> ParseError<U> {
+    pub fn map<T: Into<U>>(other: ParseError<T>) -> Self {
+        match other {
+            ParseError::EndOfFile => Self::EndOfFile,
+            ParseError::InvalidCharacter(c) => Self::InvalidCharacter(c),
+            ParseError::Other(t) => Self::Other(t.into())
+        }
+    }
+}
+
+/// Parse a value from an iterator.
+pub trait Parse: Sized {
+    /// The associated error that can be returned from parsing.
+    type Err;
+    /// Parses `iter` to return a value of this type.
+    ///
+    /// This method may not reach the end of `iter`. If so, it must not consume any
+    /// characters in the stream that do not form part of the value.
+    ///
+    /// If parsing succeeds, return the value inside [`Ok`]. Otherwise, when the
+    /// iterator is ill-formatted, return an error specific to the inside [`Err`].
+    /// The error type is specific to the implementation of the trait.
+    fn parse<I: PeekExt<Item = char>>(iter: I) -> Result<Self, ParseError<Self::Err>>;
+}
+
+/// Consumes the next item in `iter` if it equals `c`. Otherwise, returns an
+/// appropriate error without advancing the iterator.
+///
+/// # Examples
+/// Basic usage
+/// ```
+/// use chomp::nibble::parse::{requires, Parse, ParseError};
+/// use std::convert::Infallible;
+///
+/// let s = "hello".to_owned();
+/// let mut iter = s.chars().peekable();
+/// assert_eq!(requires::<_, Infallible>(&mut iter, 'h'), Ok(()));
+/// assert_eq!(requires::<_, Infallible>(&mut iter, 'e'), Ok(()));
+/// assert_eq!(requires::<_, Infallible>(&mut iter, 'z'), Err(ParseError::InvalidCharacter('l')));
+/// assert_eq!(iter.next(), Some('l'));
+/// assert_eq!(iter.next(), Some('l'));
+/// assert_eq!(iter.next(), Some('o'));
+/// assert_eq!(requires::<_, Infallible>(iter, '!'), Err(ParseError::EndOfFile));
+/// ```
+pub fn requires<I: Peek<Item = char>, E>(mut iter: I, c: char) -> Result<(), ParseError<E>> {
+    iter.next_if_eq(&c)
+        .ok_or(ParseError::EndOfFile)?
+        .map_err(|&c| ParseError::InvalidCharacter(c))
+}