# Compatibility of alternative representation of Expressions with SymbolicUtils.jl

**URL:** <https://discourse.julialang.org/t/compatibility-of-alternative-representation-of-expressions-with-symbolicutils-jl/123707>\
**Category:** Modelling & Simulations\
**Created:** [December 11, 2024, 1:17pm UTC](https://discourse.julialang.org/t/compatibility-of-alternative-representation-of-expressions-with-symbolicutils-jl/123707 "2024-12-11T13:17:06Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![Tomas\_Pevny](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tomas_pevny/32/25466_2.png) [@Tomas\_Pevny](https://discourse.julialang.org/u/Tomas_Pevny)\
**Post date:** [December 11, 2024, 1:17pm UTC](https://discourse.julialang.org/t/compatibility-of-alternative-representation-of-expressions-with-symbolicutils-jl/123707/1 "2024-12-11T13:17:07Z")

</div>

Hello,

with my student we are trying to reproduce this Facebook’s paper ([GitHub - facebookresearch/neural-rewriter: Learning to Perform Local Rewriting for Combinatorial Optimization](https://github.com/facebookresearch/neural-rewriter)) in Julia. At the moment, we use Metatheory.jl for matching and rule system, but since we are operating over julia native `Expr`essions, the code is not type stable and hence slow. Therefore we have started to experiment with an alternative representation of `Expr`, which are not as expressive (restricted to two childs at most), but for this project it is sufficient. The idea is that we represent each `Expr` as a structure

```julia
struct OnlyNode
    head::Symbol
    iscall::Bool
    v::Float32
    left::NodeID
    right::NodeID
end

```

where `NodeID` is just an integer identifying a concrete expressions. With that, each expression is represented by a unique `NodeID` and if it has as children subexpressions, it they are `NodeIDs` as well. I expect I have implemented something which is known to the community.

My question now is if I can make this compatible with rule system of `SymbolicUtils.jl`. I have extended the `TermInterface.jl` which on the example from doc would behave like this

I put the expression to the rewriting system

```julia
nc = NodeCache()
julia> ex = get!(nc, :(f(a, b)))
NodeID(0x000003ca)

```

and the term interface will behave like this

```julia
julia> head(ex)
:call

julia> children(ex)
3-element Vector{Any}:
 :f
 NodeID(0x000003c8)
 NodeID(0x000003c9)

julia> operation(ex)
:f

julia> arguments(ex)
2-element Vector{NodeID}:
 NodeID(0x000003c8)
 NodeID(0x000003c9)

julia> isexpr(ex)
true

julia> iscall(ex)
true

```

But it does not work with the rule rewriting system. For example this

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

```

Now if I try the example from SymbolicUtils

```julia
julia> ex = get!(nc, :(sin(x + y)))
NodeID(0x000003ce)

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

julia> r(ex)

```

it return nothing. Can someone suggests what I do incorrectly?

Thanks a lot in advance,  
Tomas

---

<div class="post-metadata">

**Author:** ![Tomas\_Pevny](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tomas_pevny/32/25466_2.png) [@Tomas\_Pevny](https://discourse.julialang.org/u/Tomas_Pevny)\
**Post date:** [December 11, 2024, 3:50pm UTC](https://discourse.julialang.org/t/compatibility-of-alternative-representation-of-expressions-with-symbolicutils-jl/123707/2 "2024-12-11T15:50:52Z")

</div>

Tracking down the problem, I think it is on the end a bit simple, but I do not know, how to resolve it.

My representation is working with symbolics.  
Consider this example

```julia
@syms x y;
nc = NodeCache();
sx = sin(x + y);
ex = get!(nc, :(sin(x + y)));

```

now `head(sx)` returns a function `sin`, whereas my representation returns Symbol `:sin`. If I construct a rule

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

```

it matches function `sin` and not Symbol `:sin`. This makes of course perfect sense. But I do not know, if there is an easy fix. I can probably redefine isequal as for example

```julia
Base.isequal(x::Symbol, ::typeof(sin)) = x == :sin

```

Is this the right solution