[ITensor] [ENHANCEMENT] Generalize ITensor constructor a little (#1039)

NDTensors/src/generic_tensor_operations.jl

@@ -32,7 +32,9 @@ end
 # Trait returning true if the two tensors or storage types can
 # contract with each other.
 @traitdef CanContract{X,Y}
-@traitimpl CanContract{X,Y} < -can_contract(X, Y)
+#! format: off
+@traitimpl CanContract{X,Y} <- can_contract(X, Y)
+#! format: on
 
 # Assume storage types can contract with each other
 can_contract(T1::Type, T2::Type) = true

src/indexset.jl

@@ -113,7 +113,9 @@ NDTensors.dim(is::IndexSet, pos::Int) = dim(is[pos])
 Return a new Indices with the indices daggered (flip
 all of the arrow directions).
 """
-dag(is::Indices) = map(i -> dag(i), is)
+function dag(is::Indices)
+  return isempty(is) ? is : map(i -> dag(i), is)
+end
 
 # TODO: move to NDTensors
 NDTensors.dim(is::Tuple, pos::Integer) = dim(is[pos])
@@ -539,6 +541,10 @@ function replaceinds(is::Indices, rep_inds::Pair{<:Index,<:Index}...)
   return replaceinds(is, zip(rep_inds...)...)
 end
 
+# Handle case of empty indices being replaced
+replaceinds(is::Indices) = is
+replaceinds(is::Indices, rep_inds::Tuple{}) = is
+
 function replaceinds(is::Indices, rep_inds::Vector{<:Pair{<:Index,<:Index}})
   return replaceinds(is, rep_inds...)
 end

src/itensor.jl

@@ -337,7 +337,7 @@ function ITensor(
   as::AliasStyle,
   eltype::Type{<:Number},
   A::AbstractArray{<:Number},
-  inds::Indices{Index{Int}};
+  inds::Indices;
   kwargs...,
 )
   length(A) ≠ dim(inds) && throw(
@@ -349,6 +349,16 @@ function ITensor(
   return itensor(Dense(data), inds)
 end
 
+function ITensor(
+  as::AliasStyle, eltype::Type{<:Number}, A::AbstractArray{<:Number}, inds; kwargs...
+)
+  is = indices(inds)
+  if !isa(is, Indices)
+    error("Indices $inds are not valid for constructing an ITensor.")
+  end
+  return ITensor(as, eltype, A, is; kwargs...)
+end
+
 # Convert `Adjoint` to `Matrix`
 function ITensor(
   as::AliasStyle, eltype::Type{<:Number}, A::Adjoint, inds::Indices{Index{Int}}; kwargs...

test/indexset.jl

@@ -364,6 +364,13 @@ using Compat
       end
     end
   end
+
+  @testset "dag" begin
+    is = [Index(2), Index(3)]
+    @test is == dag(is)
+    is = Index[]
+    @test dag(is) == Index[]
+  end
 end
 
 nothing

test/itensor.jl

@@ -835,6 +835,25 @@ end
     T[i => 1, j => 1] = 3.3
     @test M[1, 1] == 1
     @test T[i => 1, j => 1] == 3.3
+
+    # Empty indices
+    A = randn(1)
+    T = itensor(A, Index[])
+    @test A[] == T[]
+    T = itensor(A, Index[], Index[])
+    @test A[] == T[]
+    T = itensor(A, Any[])
+    @test A[] == T[]
+
+    A = randn(1, 1)
+    T = itensor(A, Index[])
+    @test A[] == T[]
+    T = itensor(A, Index[], Index[])
+    @test A[] == T[]
+    T = itensor(A, Any[], Any[])
+    @test A[] == T[]
+
+    @test_throws ErrorException itensor(rand(1), Int[1])
   end
 
   @testset "Construct from AbstractArray" begin
@@ -1138,6 +1157,10 @@ end
       @test hasinds(rA1, s2, l, l')
       @test hasinds(A1, s1, l, l')
 
+      @test replaceinds(A1, [] => []) == A1
+      @test replaceinds(A1, ()) == A1
+      @test replaceinds(A1) == A1
+
       # Pair notation (like Julia's replace function)
       rA1 = replaceind(A1, s1 => s2)
       @test hasinds(rA1, s2, l, l')