StackOverflowError when contracting with specified cutoff

ITensor Julia Questions
1

I’ve been coming across a strange problem in ITensors v0.3.17.

s = [Index(2) for i in 1:3]
a = randomITensor(s[1], s[2])
b = randomITensor(s[2], s[3])

Contracting these two tensors without specifying the cutoff keyword works fine:

julia> contract([a, b])
ITensor ord=2 (dim=2|id=803) (dim=2|id=272)
NDTensors.Dense{Float64, Vector{Float64}}

But specifying the cutoff seems to make it infinitely recurse:

julia> contract([a, b]; cutoff=1e-13)
ERROR: StackOverflowError:
Stacktrace:
     [1] contract(::ITensor, ::Vararg{ITensor}; kwargs::Base.Pairs{Symbol, Float64, Tuple{Symbol}, NamedTuple{(:cutoff,), Tuple{Float64}}})
       @ ITensors ~/.julia/packages/ITensors/MUEOu/src/itensor.jl:2060
     [2] (::ITensors.var"#233#235"{Base.Pairs{Symbol, Float64, Tuple{Symbol}, NamedTuple{(:cutoff,), Tuple{Float64}}}})(A::ITensor, B::ITensor)
       @ ITensors ~/.julia/packages/ITensors/MUEOu/src/itensor.jl:2050
     [3] (::Base.BottomRF{ITensors.var"#233#235"{Base.Pairs{Symbol, Float64, Tuple{Symbol}, NamedTuple{(:cutoff,), Tuple{Float64}}}}})(acc::ITensor, x::ITensor)
       @ Base ./reduce.jl:81
     [4] afoldl(::Base.BottomRF{ITensors.var"#233#235"{Base.Pairs{Symbol, Float64, Tuple{Symbol}, NamedTuple{(:cutoff,), Tuple{Float64}}}}}, ::Base._InitialValue, ::ITensor, ::ITensor)
       @ Base ./operators.jl:613
     [5] _foldl_impl(op::Base.BottomRF{ITensors.var"#233#235"{Base.Pairs{Symbol, Float64, Tuple{Symbol}, NamedTuple{(:cutoff,), Tuple{Float64}}}}}, init::Base._InitialValue, itr::Tuple{ITensor, ITensor})
       @ Base ./tuple.jl:277
     [6] foldl_impl(op::Base.BottomRF{ITensors.var"#233#235"{Base.Pairs{Symbol, Float64, Tuple{Symbol}, NamedTuple{(:cutoff,), Tuple{Float64}}}}}, nt::Base._InitialValue, itr::Tuple{ITensor, ITensor})
       @ Base ./reduce.jl:48
     [7] mapfoldl_impl(f::typeof(identity), op::ITensors.var"#233#235"{Base.Pairs{Symbol, Float64, Tuple{Symbol}, NamedTuple{(:cutoff,), Tuple{Float64}}}}, nt::Base._InitialValue, itr::Tuple{ITensor, ITensor})
       @ Base ./reduce.jl:44
     [8] mapfoldl(f::Function, op::Function, itr::Tuple{ITensor, ITensor}; init::Base._InitialValue)
       @ Base ./reduce.jl:162
     [9] mapfoldl
       @ ./reduce.jl:162 [inlined]
    [10] #foldl#245
       @ ./reduce.jl:180 [inlined]
    [11] foldl(op::Function, itr::Tuple{ITensor, ITensor})
       @ Base ./reduce.jl:180
    [12] contract(As::Tuple{ITensor, ITensor}; sequence::String, kwargs::Base.Pairs{Symbol, Float64, Tuple{Symbol}, NamedTuple{(:cutoff,), Tuple{Float64}}})
       @ ITensors ~/.julia/packages/ITensors/MUEOu/src/itensor.jl:2050
--- the last 12 lines are repeated 4703 more times ---

Looking at line 2050 in ITensors.jl,

    return foldl((A, B) -> contract(A, B; kwargs...), As)

it isn’t immediately obvious to me how this could cause the infinite recursion. Am I using contract incorrectly somehow?

2

I’m not sure why adding that keyword argument causes an infinite recursion, but cutoff isn’t supported for contracting ITensors anyway since approximate contraction of ITensors is not currently support through contract.

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