Merge pull request #78 from RelationalAI-oss/axb-derived-function-can-have-a-return-type-2026-01-14

Allow @derived function to have a return type annotation

Author: bergel

src/linting/extended_checks.jl

@@ -889,6 +889,11 @@ function check(t::ReturnTypeAnnotationRule, x::EXPR, markers::Dict{Symbol,String
         contains(markers[:filename], "test.jl") && return
     end
 
+    if haskey(markers, :macrocall)
+        # return type annotation are mandatory in @derived functions
+        contains(markers[:macrocall], "derived") && return
+    end
+
     msg = "Avoid return type annotations `function foo()::Type`. Return type annotations can hurt performance by forcing type conversions. [Explanation](https://github.com/RelationalAI/RAIStyle?tab=readme-ov-file#type-annotations)."
 
     # Pattern: function name()::Type ... end (multiline)

test/rai_rules_tests.jl

@@ -2090,6 +2090,22 @@ end
         @test lint_test(source,
             "Line 1, column 1: Avoid return type annotations")
     end
+
+    @testset "one-liner without return type" begin
+        source = """
+            foo(x::Int) = string(x)
+            """
+        @test !lint_has_error_test(source)
+    end
+
+    @testset "derived functions" begin
+        source = """
+            @derived v=1 function _is_ivm_frame(rt::Runtime, path::RelPath)::Bool
+                return RAI_BackIR.has_attr(definition_of(rt, path), :frame)
+            end
+            """
+        @test !lint_has_error_test(source)
+    end
 end
 
 @testset "String concatenation with *" begin