fix: add support for comprehensions with conditionals

Comprehension support works by parsing the underlying generators, and
generators with `if` conditions have `filter` as their `Expr` form's
second argument. This previously wasn't supported, as the generator
parsing code expected the `for` iteration's `=` to be the second arg
instead. 

This change adds support for the case of generators with filters, and
adds relevant tests.

Author: digital-carver

CHANGELOG.md

@@ -1,5 +1,9 @@
 # News
 
+## v1.0.4 - 2025-08-16
+
+- Support comprehensions with conditionals, e.g., `@resumable function f1(); [i for i in 1:10 if i<5]; end` which previously led to "Illegal expression" errors.
+
 ## v1.0.3 - 2025-03-24
 
 - Internal changes to `fsmi_generator` to support julia 1.12

src/utils.jl

@@ -338,19 +338,31 @@ function scoping(s::Symbol, scope; new = false)
   return lookup_rhs!(s, scope)
 end
 
-function scope_generator(expr, scope)
-  expr.head !== :generator && error("Illegal generator expression: $(expr)")
-  # first the generator case
+function scope_generator_inner(expr, scope)
   for i in 2:length(expr.args)
-    !(expr.args[i] isa Expr && expr.args[i].head === :(=)) && error("Illegal expression in generator: $(expr.args[i])")
+    !(expr.args[i] isa Expr && expr.args[i].head === :(=)) &&
+      error("Illegal expression in generator: $(expr.args[i])")
     expr.args[i].args[2] = scoping(expr.args[i].args[2], scope)
   end
   # now create new scope
   push!(scope.scope_stack, Dict())
   for i in 2:length(expr.args)
     expr.args[i].args[1] = lookup_lhs!(expr.args[i].args[1], scope, new = true)
   end
+end
+
+function scope_generator(expr, scope)
+  expr.head !== :generator && error("Illegal generator expression: $(expr)")
 
+  has_filter = length(expr.args) == 2 && expr.args[2] isa Expr && expr.args[2].head === :filter
+  if has_filter
+    ex = expr.args[2]
+    scope_generator_inner(ex, scope)
+    # now apply scoping to the filter condition expression
+    ex.args[1] = scoping(ex.args[1], scope)
+  else
+    scope_generator_inner(expr, scope)
+  end
   expr.args[1] = scoping(expr.args[1], scope)
   pop!(scope.scope_stack)
   return expr
@@ -432,10 +444,6 @@ function scoping(expr::Expr, scope)
     end
     return expr
   end
-  if expr.head === :generator
-    expr = scope_generator(expr)
-    return expr
-  end
 
   if expr.head === :(=)
     # One special case, where we need to have both LHS and RHS at our hands

test/test_main.jl

@@ -403,6 +403,26 @@ end
     end
   end
   @test collect(test_comprehension5()) == [2, 3, 4, 3, 4, 5]
+
+  @resumable function test_comprehension_if()
+    @yield [i for i in 1:10 if i < 5]
+  end
+  @test collect(test_comprehension_if()) == [[1, 2, 3, 4]]
+
+  @resumable function test_comprehension_if_multi_iterators()
+    @yield [i + j for i in 1:3, j in 1:3 if i <= j]
+  end
+  @test collect(test_comprehension_if_multi_iterators()) == [[2, 3, 4, 4, 5, 6]]
+
+  @resumable function test_comprehension_if_nested_for()
+    @yield [i * j for i in 1:3 for j in 1:3 if i * j < 5]
+  end
+  @test collect(test_comprehension_if_nested_for()) == [[1, 2, 3, 2, 4, 3]]
+
+  @resumable function test_comprehension_if_mixed()
+    @yield [i + j + k for i in 1:2, j in 1:2 for k in 1:2 if i + j + k == 4]
+  end
+  @test collect(test_comprehension_if_mixed()) == [[4, 4, 4]]
 end
 
 @testset "test_ref" begin