Add lazy and symbolic ITensors with contraction-order optimization#182
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## Summary Adds the lazy and symbolic tensor types and the contraction-order machinery to ITensorBase, in the top-level namespace: `LazyITensor` and `SymbolicITensor` expression leaves, `lazy` / `Mul` / `symnameddims` / `substitute` for building lazy contraction expressions, and `optimize_evaluation_order` with the `Greedy` and `Optimal` order algorithms. This code previously lived in ITensorNetworksNext's `LazyITensors` submodule. It moves down to ITensorBase so it sits alongside the `ITensor` type it builds on, and a follow-up will switch ITensorNetworksNext to consume it instead of carrying its own copy. `AbstractTrees` becomes a regular dependency rather than a weakdep: the lazy and symbolic `show` and tree rendering use it throughout, and the `printnode(::AbstractITensor)` method that was in `ITensorBaseAbstractTreesExt` moves into the package. `TermInterface`, `WrappedUnions`, and `Combinatorics` are new dependencies. The `Optimal` order, which uses `TensorOperations.optimaltree`, is provided by a new `TensorOperations` extension. `SymbolicITensor` stores `name`, `denamed_axes`, and `dimnames`, mirroring how a plain `ITensor` splits into `denamed` and `dimnames`. The symbolic leaf has no array, so it keeps the underlying axes directly. It is parameterized by the dimension-name type and the name type, the latter matching how the tensor network types parameterize by vertex type.
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## main #182 +/- ##
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- Coverage 74.75% 71.05% -3.70%
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Lines 1105 1451 +346
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- Misses 279 420 +141
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A symbolic tensor is substituted with a real tensor before contraction,
so it only needs the index sizes for contraction-order selection. Store
`size::Vector{Int}` alongside `dimnames` and reconstruct `inds` as plain
ranges of those sizes.
mtfishman
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to ITensor/ITensorNetworksNext.jl
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Jun 22, 2026
## Summary Switches ITensorNetworksNext to use the lazy and symbolic ITensor layer from ITensorBase instead of its own copy. The `LazyITensors` submodule is removed, and its lazy and symbolic types and contraction-order machinery are imported directly from ITensorBase, where the implementation moved in ITensor/ITensorBase.jl#182. Also drops the now-redundant `ITensorNetworksNextTensorOperationsExt` (the `Optimal` order via `TensorOperations.optimaltree` is provided by ITensorBase's own extension) and the `AbstractTrees`, `TermInterface`, and `WrappedUnions` dependencies that only the local copy used.
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Summary
Adds the lazy and symbolic tensor types and the contraction-order machinery to ITensorBase, in the top-level namespace:
LazyITensorandSymbolicITensorexpression leaves,lazy/Mul/symnameddims/substitutefor building lazy contraction expressions, andoptimize_evaluation_orderwith theGreedyandOptimalorder algorithms. This code was moved from ITensorNetworksNext'sLazyITensorssubmodule, most recently reworked in ITensor/ITensorNetworksNext.jl#123 to use dedicated contraction-order algorithm types and fewer dependencies. It moves down to ITensorBase so it sits alongside theITensortype it builds on, and a follow-up will switch ITensorNetworksNext to consume it instead of carrying its own copy.AbstractTreesbecomes a regular dependency rather than a weakdep: the lazy and symbolicshowand tree rendering use it throughout, and theprintnode(::AbstractITensor)method that was inITensorBaseAbstractTreesExtmoves into the package.TermInterface,WrappedUnions, andCombinatoricsare new dependencies. TheOptimalorder, which usesTensorOperations.optimaltree, is provided by a newTensorOperationsextension.SymbolicITensoris a placeholder that gets substituted with a real tensor before contraction, so it stores only what contraction-order selection needs:name, the indexsizes, anddimnames, withindsreconstructed as plain ranges of those sizes. It is parameterized by the dimension-name type and the name type, the latter matching how the tensor network types parameterize by vertex type.