API Reference

@syms <lhs_expr>[::T1] <lhs_expr>[::T2]...

For instance:

@syms foo::Real bar baz(x, y::Real)::Complex

Create one or more variables. <lhs_expr> can be just a symbol in which case it will be the name of the variable, or a function call in which case a function-like variable which has the same name as the function being called. The Sym type, or in the case of a function-like Sym, the output type of calling the function can be set using the ::T syntax.

Examples:

• @syms foo bar::Real baz::Int will create

variable foo of symtype Number (the default), bar of symtype Real and baz of symtype Int

• @syms f(x) g(y::Real, x)::Int h(a::Int, f(b)) creates 1-arg f 2-arg g

and 2 arg h. The second argument to h must be a one argument function-like variable. So, h(1, g) will fail and h(1, f) will work.

promote_symtype(f, Ts...)

The result of applying f to arguments of symtype Ts...

julia> promote_symtype(+, Real, Real)
Real

julia> promote_symtype(+, Complex, Real)
Number

julia> @syms f(x)::Complex
(f(::Number)::Complex,)

julia> promote_symtype(f, Number)
Complex

When constructing Terms without an explicit symtype, promote_symtype is used to figure out the symtype of the Term.

promote_symtype(f::FnType{X,Y}, arg_symtypes...)

The output symtype of applying variable f to arguments of symtype arg_symtypes.... if the arguments are of the wrong type then this function will error.

@rule LHS => RHS

Creates a Rule object. A rule object is callable, and takes an expression and rewrites it if it matches the LHS pattern to the RHS pattern, returns nothing otherwise. The rule language is described below.

LHS can be any possibly nested function call expression where any of the arguments can optionally be a Slot (~x) or a Segment (~~x) (described below).

If an expression matches LHS entirely, then it is rewritten to the pattern in the RHS Segment (~x) and slot variables (~~x) on the RHS will substitute the result of the matches found for these variables in the LHS.

Slot:

A Slot variable is written as ~x and matches a single expression. x is the name of the variable. If a slot appears more than once in an LHS expression then expression matched at every such location must be equal (as shown by isequal).

Example:

Simple rule to turn any sin into cos:

julia> @syms a b c
(a, b, c)

julia> r = @rule sin(~x) => cos(~x)
sin(~x) => cos(~x)

julia> r(sin(1+a))
cos((1 + a))

A rule with 2 segment variables

julia> r = @rule sin(~x + ~y) => sin(~x)*cos(~y) + cos(~x)*sin(~y)
sin(~x + ~y) => sin(~x) * cos(~y) + cos(~x) * sin(~y)

julia> r(sin(a + b))
cos(a)*sin(b) + sin(a)*cos(b)

A rule that matches two of the same expressions:

julia> r = @rule sin(~x)^2 + cos(~x)^2 => 1
sin(~x) ^ 2 + cos(~x) ^ 2 => 1

julia> r(sin(2a)^2 + cos(2a)^2)
1

julia> r(sin(2a)^2 + cos(a)^2)
# nothing

Segment:

A Segment variable is written as ~~x and matches zero or more expressions in the function call.

Example:

This implements the distributive property of multiplication: +(~~ys) matches expressions like a + b, a+b+c and so on. On the RHS ~~ys presents as any old julia array.

julia> r = @rule ~x * +((~~ys)) => sum(map(y-> ~x * y, ~~ys));

julia> r(2 * (a+b+c))
((2 * a) + (2 * b) + (2 * c))

Predicates:

There are two kinds of predicates, namely over slot variables and over the whole rule. For the former, predicates can be used on both ~x and ~~x by using the ~x::f or ~~x::f. Here f can be any julia function. In the case of a slot the function gets a single matched subexpression, in the case of segment, it gets an array of matched expressions.

The predicate should return true if the current match is acceptable, and false otherwise.

julia> two_πs(x::Number) = abs(round(x/(2π)) - x/(2π)) < 10^-9
two_πs (generic function with 1 method)

julia> two_πs(x) = false
two_πs (generic function with 2 methods)

julia> r = @rule sin(~~x + ~y::two_πs + ~~z) => sin(+(~~x..., ~~z...))
sin(~(~x) + ~(y::two_πs) + ~(~z)) => sin(+(~(~x)..., ~(~z)...))

julia> r(sin(a+3π))

julia> r(sin(a+6π))
sin(a)

julia> r(sin(a+6π+c))
sin((a + c))

Predicate function gets an array of values if attached to a segment variable (~~x).

For the predicate over the whole rule, use @rule <LHS> => <RHS> where <predicate>:

julia> @syms a b;

julia> predicate(x) = x === a;

julia> r = @rule ~x => ~x where f(~x);

julia> r(a)
a

julia> r(b) === nothing
true

Note that this is syntactic sugar and that it is the same as something like @rule ~x => f(~x) ? ~x : nothing.

Context:

In predicates: Contextual predicates are functions wrapped in the Contextual type. The function is called with 2 arguments: the expression and a context object passed during a call to the Rule object (maybe done by passing a context to simplify or a RuleSet object).

The function can use the inputs however it wants, and must return a boolean indicating whether the predicate holds or not.

In the consequent pattern: Use (@ctx) to access the context object on the right hand side of an expression.

A rewriter is any function which takes an expression and returns an expression or nothing. If nothing is returned that means there was no changes applicable to the input expression.

The Rewriters module contains some types which create and transform rewriters.

• Empty() is a rewriter which always returns nothing
• Chain(itr) chain an iterator of rewriters into a single rewriter which applies each chained rewriter in the given order. If a rewriter returns nothing this is treated as a no-change.
• RestartedChain(itr) like Chain(itr) but restarts from the first rewriter once on the first successful application of one of the chained rewriters.
• IfElse(cond, rw1, rw2) runs the cond function on the input, applies rw1 if cond returns true, rw2 if it returns false
• If(cond, rw) is the same as IfElse(cond, rw, Empty())
• Prewalk(rw; threaded=false, thread_cutoff=100) returns a rewriter which does a pre-order traversal of a given expression and applies the rewriter rw. Note that if rw returns nothing when a match is not found, then Prewalk(rw) will also return nothing unless a match is found at every level of the walk. threaded=true will use multi threading for traversal. thread_cutoff is the minimum number of nodes in a subtree which should be walked in a threaded spawn.
• Postwalk(rw; threaded=false, thread_cutoff=100) similarly does post-order traversal.
• Fixpoint(rw) returns a rewriter which applies rw repeatedly until there are no changes to be made.
• FixpointNoCycle behaves like Fixpoint but instead it applies rw repeatedly only while it is returning new results.
• PassThrough(rw) returns a rewriter which if rw(x) returns nothing will instead return x otherwise will return rw(x).

simplify(x; expand=false,
            threaded=false,
            thread_subtree_cutoff=100,
            rewriter=nothing)

Simplify an expression (x) by applying rewriter until there are no changes. expand=true applies expand in the beginning of each fixpoint iteration.

By default, simplify will assume denominators are not zero and allow cancellation in fractions. Pass simplify_fractions=false to prevent this.

expand(expr)

Expand expressions by distributing multiplication over addition, e.g., a*(b+c) becomes ab+ac.

expand uses replace symbols and non-algebraic expressions by variables of type variable_type to compute the distribution using a specialized sparse multivariate polynomials implementation. variable_type can be any subtype of MultivariatePolynomials.AbstractVariable.

substitute(expr, dict; fold=true)

substitute any subexpression that matches a key in dict with the corresponding value. If fold=false, expressions which can be evaluated won't be evaluated.

julia> substitute(1+sqrt(y), Dict(y => 2), fold=true)
2.414213562373095
julia> substitute(1+sqrt(y), Dict(y => 2), fold=false)
1 + sqrt(2)

@timerewrite expr

If expr calls simplify or a RuleSet object, track the amount of time it spent on applying each rule and pretty print the timing.

This uses TimerOutputs.jl.

Example:

julia> expr = foldr(*, rand([a,b,c,d], 100))
(a ^ 26) * (b ^ 30) * (c ^ 16) * (d ^ 28)

julia> @timerewrite simplify(expr)
 ────────────────────────────────────────────────────────────────────────────────────────────────
                                                         Time                   Allocations
                                                 ──────────────────────   ───────────────────────
                Tot / % measured:                     340ms / 15.3%           92.2MiB / 10.8%

 Section                                 ncalls     time   %tot     avg     alloc   %tot      avg
 ────────────────────────────────────────────────────────────────────────────────────────────────
 ACRule((~y) ^ ~n * ~y => (~y) ^ (~n ...    667   11.1ms  21.3%  16.7μs   2.66MiB  26.8%  4.08KiB
   RHS                                       92    277μs  0.53%  3.01μs   14.4KiB  0.14%     160B
 ACRule((~x) ^ ~n * (~x) ^ ~m => (~x)...    575   7.63ms  14.6%  13.3μs   1.83MiB  18.4%  3.26KiB
 (*)(~(~(x::!issortedₑ))) => sort_arg...    831   6.31ms  12.1%  7.59μs    738KiB  7.26%     910B
   RHS                                      164   3.03ms  5.81%  18.5μs    250KiB  2.46%  1.52KiB
   ...
   ...
 ────────────────────────────────────────────────────────────────────────────────────────────────
(a ^ 26) * (b ^ 30) * (c ^ 16) * (d ^ 28)