Update README.md

Author: MikeInnes

README.md

@@ -2,245 +2,4 @@
 
 [![Build Status](https://travis-ci.org/MikeInnes/MacroTools.jl.svg?branch=master)](https://travis-ci.org/MikeInnes/MacroTools.jl)
 
-This library provides helpful tools for writing macros, notably a very simple
-but powerful templating system and some functions that have proven useful to me (see
-[utils.jl](src/utils.jl).)
-
-## Template Matching
-
-Template matching enables macro writers to deconstruct Julia
-expressions in a more declarative way, and without having to know in
-great detail how syntax is represented internally. For example, say you
-have a type definition:
-
-```julia
-ex = quote
-  struct Foo
-    x::Int
-    y
-  end
-end
-```
-
-If you know what you're doing, you can pull out the name and fields via:
-
-```julia
-julia> if isexpr(ex.args[2], :struct)
-         (ex.args[2].args[2], ex.args[2].args[3].args)
-       end
-(:Foo,{:( # line 3:),:(x::Int),:( # line 4:),:y})
-```
-
-But this is hard to write – since you have to deconstruct the `type`
-expression by hand – and hard to read, since you can't tell at a glance
-what's being achieved. On top of that, there's a bunch of messy stuff to
-deal with like pesky `begin` blocks which wrap a single expression, line
-numbers, etc. etc.
-
-Enter MacroTools:
-
-```julia
-julia> using MacroTools
-
-julia> @capture(ex, struct T_ fields__ end)
-true
-
-julia> T, fields
-(:Foo, [:(x::Int), :y])
-```
-
-Symbols like `T_` underscore are treated as catchalls which match any
-expression, and the expression they match is bound to the
-(underscore-less) variable, as above.
-
-Because `@capture` doubles as a test as well as extracting values, you can
-easily handle unexpected input (try writing this by hand):
-
-```julia
-@capture(ex, f_{T_}(xs__) = body_) ||
-  error("expected a function with a single type parameter")
-```
-
-Symbols like `f__` (double underscored) are similar, but slurp a sequence of
-arguments into an array. For example:
-
-```julia
-julia> @capture(:[1, 2, 3, 4, 5, 6, 7], [1, a_, 3, b__, c_])
-true
-
-julia> a, b, c
-(2,[4,5,6],7)
-```
-
-Slurps don't have to be at the end of an expression, but like the
-Highlander there can only be one (per expression).
-
-### Matching on expression type
-
-`@capture` can match expressions by their type, which is either the `head` of `Expr`
-objects or the `typeof` atomic stuff like `Symbol`s and `Int`s. For example:
-
-```julia
-@capture(ex, foo(x_String_string))
-```
-
-This will match a call to the `foo` function which has a single argument, which
-may either be a `String` object or a `Expr(:string, ...)`
-(e.g. `@capture(:(foo("$(a)")), foo(x_String_string))`). Julia string literals
-may be parsed into either type of object, so this is a handy way to catch both.
-
-Another common use case is to catch symbol literals, e.g.
-
-```julia
-@capture(ex,
-  struct T_Symbol
-    fields__
-  end)
-```
-
-which will match e.g. `struct Foo ...` but not `struct Foo{V} ...`
-
-### Unions
-
-`@capture` can also try to match the expression against one pattern or another,
-for example:
-
-```julia
-@capture(ex, f_(args__) = body_ | function f_(args__) body_ end)
-```
-
-will match both kinds of function syntax (though it's easier to use
-`shortdef` to normalise definitions). You can also do this within
-expressions, e.g.
-
-```julia
-@capture(ex, (f_{T_}|f_)(args__) = body_)
-```
-
-matches a function definition, with a single type parameter bound to `T` if possible.
-If not, `T = nothing`.
-
-## Expression Walking
-
-If you've ever written any more interesting macros, you've probably found
-yourself writing recursive functions to work with nested `Expr` trees.
-MacroTools' `prewalk` and `postwalk` functions factor out the recursion, making
-macro code much more concise and robust.
-
-These expression-walking functions essentially provide a kind of
-find-and-replace for expression trees. For example:
-
-```julia
-julia> using MacroTools: prewalk, postwalk
-
-julia> postwalk(x -> x isa Integer ? x + 1 : x, :(2+3))
-:(3 + 4)
-```
-
-In other words, look at each item in the tree; if it's an integer, add one, if not, leave it alone.
-
-We can do more complex things if we combine this with `@capture`. For example, say we want to insert an extra argument into all function calls:
-
-```julia
-julia> ex = quote
-         x = f(y, g(z))
-         return h(x)
-       end
-
-julia> postwalk(x -> @capture(x, f_(xs__)) ? :($f(5, $(xs...))) : x, ex)
-quote  # REPL[20], line 2:
-    x = f(5, y, g(5, z)) # REPL[20], line 3:
-    return h(5, x)
-end
-```
-
-Most of the time, you can use `postwalk` without worrying about it, but we also
-provide `prewalk`. The difference is the order in which you see sub-expressions;
-`postwalk` sees the leaves of the `Expr` tree first and the whole expression
-last, while `prewalk` is the opposite.
-
-```julia
-julia> postwalk(x -> @show(x) isa Integer ? x + 1 : x, :(2+3*4));
-x = :+
-x = 2
-x = :*
-x = 3
-x = 4
-x = :(4 * 5)
-x = :(3 + 4 * 5)
-
-julia> prewalk(x -> @show(x) isa Integer ? x + 1 : x, :(2+3*4));
-x = :(2 + 3 * 4)
-x = :+
-x = 2
-x = :(3 * 4)
-x = :*
-x = 3
-x = 4
-```
-
-A significant difference is that `prewalk` will walk into whatever expression
-you return.
-
-```julia
-julia> postwalk(x -> @show(x) isa Integer ? :(a+b) : x, 2)
-x = 2
-:(a + b)
-
-julia> prewalk(x -> @show(x) isa Integer ? :(a+b) : x, 2)
-x = 2
-x = :+
-x = :a
-x = :b
-:(a + b)
-```
-
-This makes it somewhat more prone to infinite loops; for example, if we returned
-`:(1+b)` instead of `:(a+b)`, `prewalk` would hang trying to expand all of the
-`1`s in the expression.
-
-With these tools in hand, a useful general pattern for macros is:
-
-```julia
-macro foo(ex)
-  postwalk(ex) do x
-    @capture(x, some_pattern) || return x
-    return new_x
-  end
-end
-```
-
-## Function definitions
-
-`splitdef(def)` matches a function definition of the form
-
-```julia
-function name{params}(args; kwargs)::rtype where {whereparams}
-   body
-end
-```
-
-and returns `Dict(:name=>..., :args=>..., etc.)`. The definition can be rebuilt by
-calling `MacroTools.combinedef(dict)`, or explicitly with
-
-```julia
-rtype = get(dict, :rtype, :Any)
-all_params = [get(dict, :params, [])..., get(dict, :whereparams, [])...]
-:(function $(dict[:name]){$(all_params...)}($(dict[:args]...);
-                                            $(dict[:kwargs]...))::$rtype
-      $(dict[:body])
-  end)
-```
-
-`splitarg(arg)` matches function arguments (whether from a definition or a function call)
-such as `x::Int=2` and returns `(arg_name, arg_type, slurp, default)`. `default` is
-`nothing` when there is none. For example:
-
-```julia
-> map(splitarg, (:(f(y, a=2, x::Int=nothing, args...))).args[2:end])
-4-element Array{Tuple{Symbol,Symbol,Bool,Any},1}:
- (:y, :Any, false, nothing)  
- (:a, :Any, false, 2)        
- (:x, :Int, false, :nothing) 
- (:args, :Any, true, nothing)
-```
+MacroTools provides a library of tools for working with Julia code and expressions. This includes a powerful template-matching system and code-walking tools that let you do deep transformations of code in a few lines. See the [docs](http://mikeinnes.github.io/MacroTools.jl/) for more info.