parsing modes

src/SUMMARY.md

@@ -10,3 +10,4 @@
   - [RcChars](./exercises/rcchars.md)
 - [Chapter 2](./chapter_2.md)
   - [AnyStr](./exercises/anystr.md)
+  - [Mode](./exercises/mode.md)

src/chapter_2.md

@@ -1 +1 @@
-# Chapter 2
+# Chapter 2: Missing Requirements

src/exercises/anystr.md

@@ -1,3 +1,5 @@
+# `AnyStr`?
+
 Create something similar to [Python's `typing.AnyStr`].
 
 [Python's `typing.AnyStr`]: https://docs.python.org/3/library/typing.html#typing.AnyStr

src/exercises/bool_stream.md

@@ -1,6 +1,8 @@
-Make `test_covariance` compile by making `BoolStream<'a>` covariant `'a`. Restrictions:
+# Introducing variance to objects
+
+Make `test_covariance` compile by making `BoolStream<'a>` covariant over `'a`. Restrictions:
 - Can only change implementation details of `BoolStream` and its methods and add extra items outside of what's given, i.e. no signature/test change.
-- Changed version must behave the same way.
+- Changed version must behave the same way as the original.
 
 Consider the following code:
 ```rust,compile_fail

src/exercises/mode.md

@@ -0,0 +1,72 @@
+# Parsing modes
+Merge implementations of `ConsumesStream` and `Deterministic` for tuple.
+
+It's recommended to first solve [this exercise](./multiple_blanket.md).
+
+```rust
+trait Stream: Sized {
+    /// Try to read `n` bytes. Consumes the stream on failure.
+    fn read_n<A, E>(
+        self,
+        n: usize,
+        ok: impl FnOnce(&[u8]) -> A,
+        err: impl FnOnce(&[u8]) -> E,
+    ) -> Result<(A, Self), E>;
+
+    /// Read all bytes, consuming the stream.
+    fn read_all<A>(self, ok: impl FnOnce(&[u8]) -> A) -> A;
+}
+
+trait Parsable: Sized {
+    type Error;
+}
+
+/// Can parse any length of data.
+trait ConsumesStream: Parsable {
+    /// Try to parse the value. Can handle EOF, can ask for all data the stream has.
+    fn parse(stream: impl Stream) -> Result<Self, Self::Error>;
+
+    /// Push extra data into the value.
+    fn extend(self, data: &[u8]) -> Result<Self, Self::Error>;
+}
+
+/// Whether the parsing is terminated, is only determined by the data already parsed.
+trait Deterministic: Parsable {
+    /// Returns the stream, as it shouldn't succeed on unexpected EOF.
+    fn parse<S: Stream>(stream: S) -> Result<(Self, S), Self::Error>;
+
+    /// Always fail on extra data.
+    fn fail(data: &[u8]) -> Self::Error;
+}
+
+enum Either<L, R> {
+    Left(L),
+    Right(R),
+}
+
+impl<A: Parsable, B: Parsable> Parsable for (A, B) {
+    type Error = Either<A::Error, B::Error>;
+}
+
+impl<A: Deterministic, B: ConsumesStream> ConsumesStream for (A, B) {
+    fn parse(stream: impl Stream) -> Result<Self, Self::Error> {
+        let (a, stream) = A::parse(stream).map_err(Either::Left)?;
+        B::parse(stream).map_err(Either::Right).map(|b| (a, b))
+    }
+
+    fn extend(self, data: &[u8]) -> Result<Self, Self::Error> {
+        self.1.extend(data).map_err(Either::Right).map(|b| (self.0, b))
+    }
+}
+
+impl<A: Deterministic, B: Deterministic> Deterministic for (A, B) {
+    fn parse<S: Stream>(stream: S) -> Result<(Self, S), Self::Error> {
+        let (a, stream) = A::parse(stream).map_err(Either::Left)?;
+        B::parse(stream).map_err(Either::Right).map(|(b, stream)| ((a, b), stream))
+    }
+
+    fn fail(data: &[u8]) -> Self::Error {
+        Either::Right(B::fail(data))
+    }
+}
+```

src/exercises/multiple_blanket.md

@@ -1,13 +1,14 @@
+# Blanket, blanket, blanket
 Make this compile with the following restrictions:
-- `A1` doesn't know about/depend on `A2`.
-- `A2` doesn't know about/depend on `A1`.
-- `A1` doesn't know about/depend on `A`.
-- `A2` doesn't know about/depend on `A`.
+- **Can't edit definitions of `A`, `test`, `tes1`, `test2`; can only add code outside of those items.**
 - **Some types are `A1` but not `A2`.**
 - **Some types are `A2` but not `A1`.**
 - Some types are `A` but not `A2`.
 - Some types are `A` but not `A1`.
-- **Can't edit definitions of `A`, `test`, `tes1`, `test2`; can only add code outside of those items.**
+- `A1` doesn't know about/depend on `A2`.
+- `A2` doesn't know about/depend on `A1`.
+- `A1` doesn't know about/depend on `A`.
+- `A2` doesn't know about/depend on `A`.
 
 ```rust
 trait A {

src/exercises/rcchars.md

@@ -1,3 +1,5 @@
+# Owned [`Chars`]
+
 [`Chars`] but keeping a strong reference ([`Rc`]) to the string.
 
 [`Chars`]: https://doc.rust-lang.org/std/str/struct.Chars.html
@@ -41,10 +43,10 @@
 ```
 
 ## Solutions
-- [Solution] used in rattlescript.
+- [Implementation] used in rattlescript.
 - [Same solution] but with some extra comments.
 - [Another solution].
 
-[Solution]: https://github.com/HavenSelph/rattlescript/blob/f8bafb8b063b9bf056efb1ea14188db0624d981c/src/interpreter/value.rs#L21-L50
+[Implementation]: https://github.com/HavenSelph/rattlescript/blob/f8bafb8b063b9bf056efb1ea14188db0624d981c/src/interpreter/value.rs#L21-L50
 [Same solution]: https://gist.github.com/timotheyca/7e46c9734653b2fcbe826ea4d13b9aa0
 [Another solution]: https://gist.github.com/timotheyca/bc419e0997d6f7ea5722fd2823a78c62

src/overview.md

@@ -1,13 +1,13 @@
 # Exercises
 
-Random Rust exercises.
+Random Rust exercises. Mostly typesystem-oriented.
 
-**Warning: this is a live-edited website, errors and accidental spoilers are to be expected.**
+**Warning if you're viewing this not on github.io: this is a live-edited website, errors and accidental spoilers are to be expected.**
 
 # Solutions
 
-Currently solutions are mostly provided in a non-human-readable form in the page source.
-Those are mostly intended for testing that the exercise is even possible to solve.
+Presently, solutions are often provided in a non-human-readable (compacted+misformatted) form as hidden code.
+Those are intended for testing that the exercise is even possible to solve.
 ```rust
 # /*
 fn fix_me() {  // Greyed out because the solution changes this line.
@@ -19,7 +19,7 @@ fn do_not_change_this() {
     fixed_name()
 }
 ```
-Some exercises may not include have the replaced parts greyed out:
+Some exercises may not have the replaced parts greyed out:
 ```rust
 # mod __ {
 fn fix_me() {