Operator expression parsing

Author: mtfishman

README.md

@@ -32,8 +32,7 @@ julia> Pkg.add("QuantumOperatorDefinitions")
 ## Examples
 
 ````julia
-using QuantumOperatorDefinitions:
-  OpName, SiteType, StateName, ⊗, controlled, op, state
+using QuantumOperatorDefinitions: OpName, SiteType, StateName, ⊗, controlled, op, state
 using LinearAlgebra: Diagonal
 using SparseArrays: SparseMatrixCSC, SparseVector
 using Test: @test

examples/README.jl

@@ -37,8 +37,7 @@ julia> Pkg.add("QuantumOperatorDefinitions")
 
 # ## Examples
 
-using QuantumOperatorDefinitions:
-  OpName, SiteType, StateName, ⊗, controlled, op, state
+using QuantumOperatorDefinitions: OpName, SiteType, StateName, ⊗, controlled, op, state
 using LinearAlgebra: Diagonal
 using SparseArrays: SparseMatrixCSC, SparseVector
 using Test: @test

src/op.jl

@@ -8,6 +8,39 @@ Base.getproperty(n::OpName, name::Symbol) = getfield(params(n), name)
 OpName{N}(params) where {N} = OpName{N,typeof(params)}(params)
 OpName{N}(; kwargs...) where {N} = OpName{N}((; kwargs...))
 
+# This compiles operator expressions, such as:
+# ```julia
+# opexpr("X + Y") == OpName("X") + OpName("Y")
+# opexpr("Ry{θ=π/2}") == OpName("Ry"; θ=π/2)
+# ```
+function opexpr(n::String)
+  return opexpr(Meta.parse(n))
+end
+opexpr(n::Number) = n
+function opexpr(n::Symbol)
+  n === :im && return im
+  n === :π && return π
+  return OpName{n}()
+end
+function opexpr(ex::Expr)
+  if Meta.isexpr(ex, :call)
+    return eval(ex.args[1])(opexpr.(ex.args[2:end])...)
+  end
+  if Meta.isexpr(ex, :curly)
+    # Syntax for parametrized gates, i.e.
+    # `opexpr("Ry{θ=π/2}")`.
+    params = ex.args[2:end]
+    kwargs = Dict(
+      map(params) do param
+        @assert Meta.isexpr(param, :(=))
+        return param.args[1] => eval(param.args[2])
+      end,
+    )
+    return OpName{ex.args[1]}(; kwargs...)
+  end
+  return error("Can't parse expression $ex.")
+end
+
 OpName(s::AbstractString; kwargs...) = OpName{Symbol(s)}(; kwargs...)
 OpName(s::Symbol; kwargs...) = OpName{s}(; kwargs...)
 name(::OpName{N}) where {N} = N

test/test_basics.jl

@@ -1,4 +1,4 @@
-using QuantumOperatorDefinitions: OpName, SiteType, ⊗, expand, op, state, nsites
+using QuantumOperatorDefinitions: OpName, SiteType, ⊗, expand, op, opexpr, state, nsites
 using LinearAlgebra: Diagonal
 using Test: @test, @testset
 
@@ -118,4 +118,15 @@ const elts = (real_elts..., complex_elts...)
       end
     end
   end
+  @testset "Parsing" begin
+    # TODO: Should this be called in the `OpName`/`op` constructor?
+    @test Matrix(opexpr("X * Y")) == op("X") * op("Y")
+    @test Matrix(opexpr("X * Y + Z")) == op("X") * op("Y") + op("Z")
+    @test Matrix(opexpr("X * Y + 2 * Z")) == op("X") * op("Y") + 2 * op("Z")
+    @test Matrix(opexpr("exp(im * (X * Y + 2 * Z))")) ==
+      exp(im * (op("X") * op("Y") + 2 * op("Z")))
+    @test Matrix(opexpr("exp(im * (X ⊗ Y + Z ⊗ Z))")) ==
+      exp(im * (kron(op("X"), op("Y")) + kron(op("Z"), op("Z"))))
+    @test Matrix(opexpr("Ry{θ=π/2}")) == op("Ry"; θ=π / 2)
+  end
 end