Complete tuple destructuring with complex splatted left hand sides

The flisp implementation has several arguable-bugs here which allow us
to observe some assignments before all side effects of the right hand
side have occurred. For example

    (x, y) = (1, undefined)

assigns to `x` before throwing the `UndefVarError`.

As another example,

    let
        f() = (x = 100)
        (x, y) = (1, f())
        x
    end

leaves `x` with the value of 100 in the existing implementation.

Both these examples violate the principle that symbolic simpification
should not be observable. As a fix, we now assign the right hand sides
to temporary variables and do this even for normal identifiers on the
right hand side, ensuring the normal left-to-right evaluation order for
function arguments on the right hand side.

Author: c42f

src/desugaring.jl

@@ -67,7 +67,7 @@ function is_effect_free(ex)
     k = kind(ex)
     # TODO: metas
     is_literal(k) || is_identifier_like(ex) || k == K"Symbol" ||
-        k == K"inert" || k == K"top" || k == K"core"
+        k == K"inert" || k == K"top" || k == K"core" || k == K"Value"
     # flisp also includes `a.b` with simple `a`, but this seems like a bug
     # because this calls the user-defined getproperty?
 end
@@ -90,96 +90,122 @@ end
 # Destructuring
 
 # Convert things like `(x,y,z) = (a,b,c)` to assignments, eliminating the
-# tuple. Includes support for slurping/splatting.
+# tuple. Includes support for slurping/splatting. This function assumes that
+# `_tuple_sides_match` returns true, so the following have already been
+# checked:
+#   * There's max one `...` on the left hand side
+#   * There's max one `...` on the right hand side, in the last place, or
+#     matched with an lhs... in the last place. (required so that
+#     pairwise-matching terms from the right is valid)
+#   * Neither side has any key=val terms or parameter blocks
 #
-# If lhss and rhss are the list of terms on each side, this function assumes
-# the following have been checked:
-#   * There's only one `...` on the left hand side
-#   * Neither side has any key=val terms
-#   * _tuple_sides_match returns true
+# Tuple elimination must act /as if/ the right hand side tuple was first
+# constructed followed by destructuring. In particular, any side effects due to
+# evaluating the individual terms in the right hand side tuple must happen in
+# order.
 function tuple_to_assignments(ctx, ex)
     lhs = ex[1]
     rhs = ex[2]
+
+    # Tuple elimination aims to turn assignments between tuples into lists of assignments.
+    #
+    # However, there's a complex interplay of side effects due to the
+    # individual assignments and these can be surprisingly complicated to
+    # model. For example `(x[i], y) = (f(), g)` can contain the following
+    # surprises:
+    # * `tmp = f()` calls `f` which might throw, or modify the bindings for
+    #   `x` or `y`.
+    # * `x[i] = tmp` is lowered to `setindex!` which might throw or modify the
+    #   bindings for `x` or `y`.
+    # * `g` might throw an `UndefVarError`
+    #
+    # Thus for correctness we introduce temporaries for all right hand sides
+    # with observable side effects and ensure they're evaluated in order.
+    n_lhs = numchildren(lhs)
+    n_rhs = numchildren(rhs)
     stmts = SyntaxList(ctx)
-    end_stmts = SyntaxList(ctx)
-    elements = SyntaxList(ctx)
-    assigned = SyntaxList(ctx)
+    rhs_tmps = SyntaxList(ctx)
+    for i in 1:n_rhs
+        rh = rhs[i]
+        r = if kind(rh) == K"..."
+            rh[1]
+        else
+            rh
+        end
+        k = kind(r)
+        if is_literal(k) || k == K"Symbol" || k == K"inert" || k == K"top" ||
+                k == K"core" || k == K"Value"
+            # Effect-free and nothrow right hand sides do not need a temporary
+            # (we require nothrow because the order of rhs terms is observable
+            #  due to sequencing, thus identifiers are not allowed)
+        else
+            # Example rhs which need a temporary
+            # * `f()` - arbitrary side effects to any binding
+            # * `z`   - might throw UndefVarError
+            tmp = emit_assign_tmp(stmts, ctx, r)
+            rh = kind(rh) == K"..." ? @ast(ctx, rh, [K"..." tmp]) : tmp
+        end
+        push!(rhs_tmps, rh)
+    end
 
     il = 0
     ir = 0
-    while il < numchildren(lhs)
+    while il < n_lhs
         il += 1
         ir += 1
         lh = lhs[il]
         if kind(lh) == K"..."
-            TODO(lhs, "... in tuple lhs")
-            n_lhs = numchildren(lhs)
-            n_rhs = numchildren(rhs)
-            if il == n_lhs
-                # Simple case: exactly one `...` at end of lhs. Examples:
-                #   (x, ys...) = (a,b,c)
-                #   (ys...)    = ()
-                rhs_tmp = emit_assign_tmp(stmts, ctx,
-                    @ast(ctx, rhs, [K"tuple" rhs[ir:end]...]),
-                    "rhs_tmp"
-                )
-                push!(stmts, @ast ctx ex [K"=" lh[1] rhs_tmp])
-                push!(elements, @ast ctx rhs_tmp [K"..." rhs_tmp])
-                break
-            else
-                # Exactly one lhs `...` occurs in the middle somewhere, with a
-                # general rhs which has one `...` term or at least as many
-                # non-`...` terms.
-                # Examples:
-                #   (x, ys..., z) = (a, b, c, d)
-                #   (x, ys..., z) = (a, bs...)
-                #   (xs..., y)    = (a, bs...)
-                # in this case we pairwise-match arguments from the end
-                # backward, with rhs splats falling back to the general case.
-                jl = n_lhs + 1
-                jr = n_rhs + 1
-                while jl > il && jr > ir
-                    if kind(lhs[jl-1]) == K"..." || kind(rhs[jr-1]) == K"..."
-                        break
-                    end
-                    jl -= 1
-                    jr -= 1
+            # Exactly one lhs `...` occurs in the middle somewhere, with a
+            # general rhs which has at least as many non-`...` terms or one
+            # `...` term at the end.
+            # Examples:
+            #   (x, ys..., z) = (a, b, c, d)
+            #   (x, ys..., z) = (a, bs...)
+            #   (xs..., y)    = (a, bs...)
+            #   (xs...) = (a, b, c)
+            # in this case we can pairwise-match arguments from the end
+            # backward and emit a general tuple assignment for the middle.
+            jl = n_lhs
+            jr = n_rhs
+            while jl > il && jr > ir
+                if kind(lhs[jl]) == K"..." || kind(rhs_tmps[jr]) == K"..."
+                    break
                 end
-                rhs[jr]
+                jl -= 1
+                jr -= 1
             end
-            continue
-        end
-        rh = rhs[ir] # In other cases `rhs[ir]` must exist
-        if kind(rh) == K"..."
-            @assert ir == numchildren(rhs) # _tuple_sides_match ensures this
-            rh_tmp = emit_assign_tmp(stmts, ctx, rh[1])
-            push!(end_stmts, @ast ctx ex [K"=" [K"tuple" lhs[il:end]...] rh_tmp])
-            push!(elements, @ast ctx rh [K"..." rh_tmp])
-            break
+            middle = emit_assign_tmp(stmts, ctx,
+                @ast(ctx, rhs, [K"tuple" rhs_tmps[ir:jr]...]),
+                "rhs_tmp"
+            )
+            if il == jl
+                # (x, ys...) = (a,b,c)
+                # (x, ys...) = (a,bs...)
+                # (ys...)    = ()
+                push!(stmts, @ast ctx ex [K"=" lh[1] middle])
+            else
+                # (x, ys..., z) = (a, b, c, d)
+                # (x, ys..., z) = (a, bs...)
+                # (xs..., y)    = (a, bs...)
+                push!(stmts, @ast ctx ex [K"=" [K"tuple" lhs[il:jl]...] middle])
+            end
+            # Continue with the remainder of the list of non-splat terms
+            il = jl
+            ir = jr
         else
-            if is_identifier_like(lh) && is_effect_free(rh) &&
-                    !any(contains_identifier(rhs[j], lh) for j in ir+1:lastindex(rhs))
-                    !any(contains_identifier(a, rh) for a in assigned)
-                # Overwrite `lh` directly if that won't cause conflicts with
-                # other symbols
-                push!(stmts, @ast ctx ex [K"=" lh rh])
-                push!(assigned, lh)
-                push!(elements, rh)
+            rh = rhs_tmps[ir]
+            if kind(rh) == K"..."
+                push!(stmts, @ast ctx ex [K"=" [K"tuple" lhs[il:end]...] rh[1]])
+                break
             else
-                # In other cases we need a temporary and we'll overwrite `lh` at the end.
-                tmp = ssavar(ctx, rh)
-                push!(stmts,     @ast ctx ex [K"=" tmp rh])
-                # `push!(assigned, lh)` is not required when we assign `lh` later.
-                push!(end_stmts, @ast ctx ex [K"=" lh  tmp])
-                push!(elements, tmp)
+                push!(stmts, @ast ctx ex [K"=" lh rh])
             end
         end
     end
 
     @ast ctx ex [K"block"
         stmts...
-        end_stmts...
-        [K"removable" [K"tuple" elements...]]
+        [K"removable" [K"tuple" rhs_tmps...]]
     ]
 end
 
@@ -354,12 +380,7 @@ function expand_property_destruct(ctx, ex)
     @assert kind(params) == K"parameters"
     rhs = ex[2]
     stmts = SyntaxList(ctx)
-    rhs1 = if is_ssa(ctx, rhs) || (is_identifier_like(rhs) &&
-                                   !any(is_same_identifier_like(l, rhs) for l in children(params)))
-        rhs
-    else
-        emit_assign_tmp(stmts, ctx, expand_forms_2(ctx, rhs))
-    end
+    rhs1 = emit_assign_tmp(stmts, ctx, expand_forms_2(ctx, rhs))
     for prop in children(params)
         propname = kind(prop) == K"Identifier"                           ? prop    :
                    kind(prop) == K"::" && kind(prop[1]) == K"Identifier" ? prop[1] :

src/eval.jl

@@ -293,7 +293,7 @@ function to_lowered_expr(mod, ex, ssa_offset=0)
                k == K"opaque_closure_method" ? :opaque_closure_method :
                nothing
         if isnothing(head)
-            TODO(ex, "Unhandled form for kind $k")
+            throw(LoweringError(ex, "Unhandled form for kind $k"))
         end
         Expr(head, map(e->to_lowered_expr(mod, e, ssa_offset), children(ex))...)
     end

src/scope_analysis.jl

@@ -58,7 +58,7 @@ function _find_scope_vars!(ctx, assignments, locals, destructured_args, globals,
         elseif kv == K"BindingId"
             binfo = lookup_binding(ctx, v)
             if !binfo.is_ssa && binfo.kind != :global
-                TODO(v, "BindingId as function name")
+                @assert false "allow local BindingId as function name?"
             end
         else
             @assert false

test/destructuring.jl

@@ -92,18 +92,61 @@ end
 end
 
 
-@testset "Tuples on both sides" begin
+@testset "Tuple elimination with tuples on both sides" begin
+
+# Simple case
+@test JuliaLowering.include_string(test_mod, """
+let a = 1, b = 2
+    (x,y) = (a,b)
+    (x,y)
+end
+""") == (1, 2)
 
 # lhs variable name in rhs
 @test JuliaLowering.include_string(test_mod, """
-let
-    x = 1
-    y = 2
+let x = 1, y = 2
     (x,y) = (y,x)
     (x,y)
 end
 """) == (2, 1)
 
+# Slurps and splats
+
+@test JuliaLowering.include_string(test_mod, """
+let a = 1, b = 2, c = 3
+    (x, ys..., z) = (a, b, c)
+    (x, ys, z)
+end
+""") == (1, (2,), 3)
+
+@test JuliaLowering.include_string(test_mod, """
+let a = 1, b = 2, cs = (3,4)
+    (x, ys...) = (a, b, cs...)
+    (x, ys)
+end
+""") == (1, (2,3,4))
+
+@test JuliaLowering.include_string(test_mod, """
+let a = 1, bs = (2,3), c = 4
+    (x, ys...) = (a, bs..., c)
+    (x, ys)
+end
+""") == (1, (2,3,4))
+
+@test JuliaLowering.include_string(test_mod, """
+let a = 1, b = 2, cs = (3,4)
+    (x, ys..., z) = (a, b, cs...)
+    (x, ys, z)
+end
+""") == (1, (2,3), 4)
+
+@test JuliaLowering.include_string(test_mod, """
+let a = 1
+    (x, ys...) = (a,)
+    (x, ys)
+end
+""") == (1, ())
+
 # dotted rhs in last place
 @test JuliaLowering.include_string(test_mod, """
 let
@@ -112,6 +155,7 @@ let
     (x,y,z)
 end
 """) == (1, 2, 3)
+
 # in value position
 @test JuliaLowering.include_string(test_mod, """
 let
@@ -120,6 +164,49 @@ let
 end
 """) == (1, 2, 3)
 
+# Side effects in the right hand tuple can affect the previous left hand side
+# bindings, for example, `x`, below. In this case we need to ensure `f()` is
+# called before `x` is assigned the value from the right hand side.
+# (the flisp implementation fails this test.)
+@test JuliaLowering.include_string(test_mod, """
+let
+   function f()
+       x=100
+       2
+   end
+   (x,y) = (1,f())
+   x,y
+end
+""") == (1,2)
+
+# `x` is not assigned and no side effect from `f()` happens when the right hand
+# side throws an UndefVarError
+@test JuliaLowering.include_string(test_mod, """
+let x=1, y=2, z=3, side_effect=false, a
+    exc = try
+        function f()
+            side_effect=true
+        end
+        (x,y,z) = (100, a, f())
+    catch e
+        e
+    end
+    (x, y, z, side_effect, exc.var)
+end
+""") == (1, 2, 3, false, :a)
+
+# Require that rhs is evaluated before any assignments, thus `x` is not defined
+# here because accessing `a` first throws an UndefVarError
+@test JuliaLowering.include_string(test_mod, """
+let x, y, a
+    try
+        (x, y) = (1, a)
+    catch
+    end
+    @isdefined(x)
+end
+""") == false
+
 end