Added an overview of chalk

Author: sunjay

src/SUMMARY.md

@@ -38,6 +38,7 @@
       - [The lowering module in rustc](./traits-lowering-module.md)
     - [Well-formedness checking](./traits-wf.md)
     - [The SLG solver](./traits-slg.md)
+    - [An Overview of Chalk](./chalk-overview.md)
     - [Bibliography](./traits-bibliography.md)
 - [Type checking](./type-checking.md)
     - [Method Lookup](./method-lookup.md)

src/chalk-overview.md

@@ -0,0 +1,140 @@
+# An Overview of Chalk
+
+> Chalk is under heavy development, so if any of these links are broken or if
+> any of the information is inconsistent with the code or outdated, please open
+> an issue so we can fix it. If you are able to fix the issue yourself, we would
+> love your contribution!
+
+[Chalk][chalk] recasts Rust's trait system explicitly in terms of logic
+programming by "lowering" Rust code into a kind of logic program we can then
+execute queries against. Its goal is to be an executable, highly readable
+specification of the Rust trait system.[^negativechalk]
+
+There are many expected benefits from this work. It will consolidate our
+existing, somewhat ad-hoc implementation into something far more principled and
+expressive, which should behave better in corner cases, and be much easier to
+extend.[^negativechalk]
+
+## Resources
+
+* [Chalk Source Code](https://github.com/rust-lang-nursery/chalk)
+* [Chalk Glossary](https://github.com/rust-lang-nursery/chalk/blob/master/GLOSSARY.md)
+* The traits section of the rustc guide (you are here)
+
+### Blog Posts
+
+* [Lowering Rust traits to logic](http://smallcultfollowing.com/babysteps/blog/2017/01/26/lowering-rust-traits-to-logic/)
+* [Unification in Chalk, part 1](http://smallcultfollowing.com/babysteps/blog/2017/03/25/unification-in-chalk-part-1/)
+* [Unification in Chalk, part 2](http://smallcultfollowing.com/babysteps/blog/2017/04/23/unification-in-chalk-part-2/)
+* [Negative reasoning in Chalk](http://aturon.github.io/blog/2017/04/24/negative-chalk/)
+* [Query structure in chalk](http://smallcultfollowing.com/babysteps/blog/2017/05/25/query-structure-in-chalk/)
+* [Cyclic queries in chalk](http://smallcultfollowing.com/babysteps/blog/2017/09/12/tabling-handling-cyclic-queries-in-chalk/)
+* [An on-demand SLG solver for chalk](http://smallcultfollowing.com/babysteps/blog/2018/01/31/an-on-demand-slg-solver-for-chalk/)
+
+## Parsing
+
+Chalk is designed to be incorporated with the Rust compiler, so the syntax and
+concepts it deals with heavily borrow from Rust. It is convenient for the sake
+of testing to be able to run chalk on its own, so chalk includes a parser for a
+Rust-like syntax.
+
+The parser takes that syntax and produces an [Abstract Syntax Tree (AST)][ast].
+You can find the [complete definition of the AST][chalk-ast] in the source code.
+
+The syntax contains things from Rust that we know and love for example traits,
+impls, and struct definitions. Parsing is often the first "phase" of
+transformation that a program goes through in order to become a format that
+chalk can understand.
+
+## Lowering
+
+After parsing, there is a "lowering" phase. This aims to convert traits/impls
+into "program clauses". A [`ProgramClause` (source code)][programclause] is
+essentially one of the following:
+
+* A [clause] of the form `consequence :- conditions` where `:-` is read as
+  "if" and `conditions = cond1 && cond2 && ...`
+* A universally quantified clause of the form `forall<T> { consequence :- conditions }`
+  * `forall<T> { ... }` is used to represent [universal quantification]. See the
+    section on [Lowering to logic][lowering-forall] for more information.
+  * A key thing to note about `forall` is that we don't allow you to "quantify"
+    over traits, only types and regions (lifetimes). That is, you can't make a
+    rule like `forall<Trait> { u32: Trait }` which would say "`u32` implements
+    all traits". You can however say `forall<T> { T: Trait }` meaning "`Trait`
+    is implemented by all types".
+  * `forall<T> { ... }` is represented in the code using the [`Binders<T>`
+    struct][binders-struct].
+
+This is the phase where we encode the rules of the trait system into logic. For
+example, if we have:
+
+```rust
+impl<T: Clone> Clone for Vec<T> {}
+```
+
+We generate:
+
+```rust
+forall<T> { (Vec<T>: Clone) :- (T: Clone) }
+```
+
+This rule dictates that `Vec<T>: Clone` is only satisfied if `T: Clone` is also
+satisfied (i.e. "provable").
+
+### Well-formedness checks
+
+As part of lowering from the AST to the internal IR, we also do some "well
+formedness" checks. See the [source code][well-formedness-checks] for where
+those are done. The call to `record_specialization_priorities` checks
+"coherence" which means that it ensures that two impls of the same trait for the
+same type cannot exist.
+
+## Intermediate Representation (IR)
+
+The second intermediate representation in chalk is called, well, the "ir". :)
+The [IR source code][ir-code] contains the complete definition. The
+`ir::Program` struct contains some "rust things" but indexed and accessible in
+a different way. This is sort of analogous to the [HIR] in Rust.
+
+For example, if you have a type like `Foo<Bar>`, we would represent `Foo` as a
+string in the AST but in `ir::Program`, we use numeric indices (`ItemId`).
+
+In addition to `ir::Program` which has "rust-like things", there is also
+`ir::ProgramEnvironment` which is "pure logic". The main field in that is
+`program_clauses` which contains the `ProgramClause`s that we generated
+previously.
+
+## Rules
+
+The `rules` module works by iterating over every trait, impl, etc. and emitting
+the rules that come from each one. The traits section of the rustc-guide (that
+you are currently reading) contains the most up-to-date reference on that.
+
+The `ir::ProgramEnvironment` is created [in this module][rules-environment].
+
+## Testing
+
+TODO: Basically, [there is a macro](https://github.com/rust-lang-nursery/chalk/blob/17abbabe53c2f78b04af04a9bc9e8a0e3fc676e3/src/solve/test/mod.rs#L112-L148)
+that will take syntax and run it through the full pipeline described above.
+[This](https://github.com/rust-lang-nursery/chalk/blob/17abbabe53c2f78b04af04a9bc9e8a0e3fc676e3/src/solve/test/mod.rs#L83-L110)
+is the function that is ultimately called.
+
+## Solver
+
+See [The SLG Solver][slg].
+
+[^negativechalk]: [*Negative reasoning in Chalk* by Aaron Turon](http://aturon.github.io/blog/2017/04/24/negative-chalk/)
+
+[chalk]: https://github.com/rust-lang-nursery/chalk
+[ast]: https://en.wikipedia.org/wiki/Abstract_syntax_tree
+[chalk-ast]: https://github.com/rust-lang-nursery/chalk/blob/master/chalk-parse/src/ast.rs
+[universal quantification]: https://en.wikipedia.org/wiki/Universal_quantification
+[lowering-forall]: traits-lowering-to-logic.html#type-checking-generic-functions-beyond-horn-clauses
+[programclause]: https://github.com/rust-lang-nursery/chalk/blob/17abbabe53c2f78b04af04a9bc9e8a0e3fc676e3/src/ir/mod.rs#L721
+[clause]: https://github.com/rust-lang-nursery/chalk/blob/master/GLOSSARY.md#clause
+[well-formedness-checks]: https://github.com/rust-lang-nursery/chalk/blob/17abbabe53c2f78b04af04a9bc9e8a0e3fc676e3/src/ir/lowering/mod.rs#L230-L232
+[ir-code]: https://github.com/rust-lang-nursery/chalk/blob/master/src/ir/mod.rs
+[HIR]: hir.html
+[binders-struct]: https://github.com/rust-lang-nursery/chalk/blob/17abbabe53c2f78b04af04a9bc9e8a0e3fc676e3/src/ir/mod.rs#L661
+[rules-environment]: https://github.com/rust-lang-nursery/chalk/blob/17abbabe53c2f78b04af04a9bc9e8a0e3fc676e3/src/rules/mod.rs#L9
+[slg]: traits-slg.html

src/traits-slg.md

@@ -1 +1,3 @@
 # The SLG solver
+
+TODO: <https://github.com/rust-lang-nursery/chalk/blob/master/chalk-engine/src/README.md>