Detect accumulation-like indexing

src/macro.jl

@@ -315,6 +315,13 @@ function parse_input(expr, store)
 
     if store.newarray && Z in store.arrays
         throw("can't create a new array $Z when this also appears on the right")
+    elseif Z in store.arrays
+        accumlike = intersect(store.leftind, store.shiftedind)
+        if !isempty(accumlike)
+            @warn "detected accumulation-like behaviour, this might be made an error" accumlike
+            store.avx = false
+            append!(store.unsafeleft, accumlike)  # this means no threading, but not sure that's enough
+        end
     end
 end
 
@@ -550,12 +557,7 @@ finishleftraw(leftraw, store) = map(enumerate(leftraw)) do (d,i)
     is_const(i) && store.newarray && (i != 1)  &&
         throw("can't fix indices on LHS when making a new array")
 
-    if isexpr(i, :$)
-        i.args[1] isa Symbol || throw("you can only interpolate single symbols, not $ex")
-        push!(store.scalars, i.args[1])
-        return i.args[1]
-
-    elseif isexpr(i, :call) && i.args[1] == :+ &&
+    if isexpr(i, :call) && i.args[1] == :+ &&
             length(i.args)==3 && i.args[3] == :_ # magic un-shift A[i+_, j] := ...
         i = primeindices(i.args)[2]
         i isa Symbol || throw("index ($i + _) is too complicated, sorry")
@@ -578,6 +580,10 @@ finishleftraw(leftraw, store) = map(enumerate(leftraw)) do (d,i)
         end
 
         return ex # has primes dealt with
+
+    elseif i isa Expr  # deal with A[i,$j] = ... and also A[i,$j+1] = ... 
+        ex = MacroTools_postwalk(dollarwalk(store), i)
+        return ex
     end
     i
 end

test/parsing.jl

@@ -698,6 +698,23 @@ end
        @tullio x[i] := s[i,j]  avx=false # Unexpected Pass with LV
     end
 
+    # https://github.com/mcabbott/Tullio.jl/issues/115
+    # we must detect that cumsum isn't thread-safe, but should it be illegal?
+    x = rand(Int8, 10) .+ 0
+    y = copy(x)
+    @tullio y[i] = y[i-1] + y[i]
+    @test y == cumsum(x)
+
+    z = (y=copy(x),)
+    @tullio z.y[i] = z.y[i-1] + z.y[i] # version with field access
+    @test z.y == cumsum(x)
+
+    # https://github.com/mcabbott/Tullio.jl/issues/115
+    A = zeros(Int, 3,4)
+    for r in 2:4
+        @tullio A[$r-1, c] = c + $r^2  avx=false # dollar on LHS with a shift
+    end
+    @test A == [c + r^2 for r in 2:4, c in 1:4]  # get wrong answer with LV
 end
 
 @printline