From fcb08e4c91cf8bd06db60232acf21b81c675e99e Mon Sep 17 00:00:00 2001
From: Adriano Meligrana <68152031+Tortar@users.noreply.github.com>
Date: Wed, 10 Sep 2025 03:39:31 +0200
Subject: [PATCH 1/4] Add apply function for type stable calls of functions

---
 src/LightSumTypes.jl | 57 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 57 insertions(+)

diff --git a/src/LightSumTypes.jl b/src/LightSumTypes.jl
index 3ea8ab1..5cc3232 100644
--- a/src/LightSumTypes.jl
+++ b/src/LightSumTypes.jl
@@ -194,6 +194,63 @@ is_sumtype(T::Type) = false
 
 function variant_idx end
 
+                    function _is_sumtype_structurally(T)
+    return T isa DataType && fieldcount(T) == 1 && fieldname(T, 1) === :variants && fieldtype(T, 1) isa Union
+end
+
+function _get_variant_types(T_sum)
+    field_T = fieldtype(T_sum, 1)
+        
+    !(field_T isa Union) && return [field_T]
+
+    types = []
+    curr = field_T
+    while curr isa Union
+        push!(types, curr.a)
+        curr = curr.b
+    end
+    push!(types, curr)
+    return types
+end
+
+@generated function apply(f::F, args::Tuple) where {F}
+    
+
+    args = fieldtypes(args)
+    sumtype_args = [(i, T) for (i, T) in enumerate(args) if _is_sumtype_structurally(T)]
+
+    if isempty(sumtype_args)
+        return :(f(args...))
+    end
+
+    final_args = Any[:(args[$i]) for i in 1:length(args)]
+    for (idx, T) in sumtype_args
+        final_args[idx] = Symbol("v_", idx)
+    end
+    
+    body = :(f($(final_args...)))
+    
+    for (idx, T) in reverse(sumtype_args)
+        unwrapped_var = Symbol("v_", idx)
+        
+        variant_types = _get_variant_types(T)
+        
+        branch_expr = :(error("THIS_SHOULD_BE_UNREACHABLE"))
+        for V_type in reverse(variant_types)
+            condition = :($unwrapped_var isa $V_type)
+            branch_expr = Expr(:elseif, condition, body, branch_expr)
+        end
+        branch_expr = Expr(:if, branch_expr.args...)
+        
+        body = quote
+            let $(unwrapped_var) = $LightSumTypes.unwrap(args[$idx])
+                $branch_expr
+            end
+        end
+    end
+    return body
+end
+
 include("precompile.jl")
 
 end

From f1c7cd2833e5a852a9a59225e798882cc0200066 Mon Sep 17 00:00:00 2001
From: Adriano Meligrana <68152031+Tortar@users.noreply.github.com>
Date: Wed, 10 Sep 2025 03:40:50 +0200
Subject: [PATCH 2/4] Add 'apply' to exported functions in LightSumTypes

---
 src/LightSumTypes.jl | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/LightSumTypes.jl b/src/LightSumTypes.jl
index 5cc3232..c251357 100644
--- a/src/LightSumTypes.jl
+++ b/src/LightSumTypes.jl
@@ -3,7 +3,7 @@ module LightSumTypes
 
 using MacroTools: namify
 
-export @sumtype, sumtype_expr, variant, variantof, allvariants, is_sumtype
+export @sumtype, sumtype_expr, variant, variantof, allvariants, is_sumtype, apply
 
 unwrap(sumt) = getfield(sumt, :variants)
 
@@ -194,7 +194,7 @@ is_sumtype(T::Type) = false
 
 function variant_idx end
 
-                    function _is_sumtype_structurally(T)
+function _is_sumtype_structurally(T)
     return T isa DataType && fieldcount(T) == 1 && fieldname(T, 1) === :variants && fieldtype(T, 1) isa Union
 end
 

From 7157817c94d38f3c439b3e16e4bf98d035660ca2 Mon Sep 17 00:00:00 2001
From: Adriano Meligrana <68152031+Tortar@users.noreply.github.com>
Date: Wed, 10 Sep 2025 04:51:46 +0200
Subject: [PATCH 3/4] Update LightSumTypes.jl

---
 src/LightSumTypes.jl | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/src/LightSumTypes.jl b/src/LightSumTypes.jl
index c251357..1a275d8 100644
--- a/src/LightSumTypes.jl
+++ b/src/LightSumTypes.jl
@@ -200,9 +200,6 @@ end
 
 function _get_variant_types(T_sum)
     field_T = fieldtype(T_sum, 1)
-        
-    !(field_T isa Union) && return [field_T]
-
     types = []
     curr = field_T
     while curr isa Union

From 5bdff85650727f9c81b7667ad051abbcfac27b2e Mon Sep 17 00:00:00 2001
From: Adriano Meligrana <68152031+Tortar@users.noreply.github.com>
Date: Wed, 10 Sep 2025 04:52:37 +0200
Subject: [PATCH 4/4] Update LightSumTypes.jl

---
 src/LightSumTypes.jl | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/src/LightSumTypes.jl b/src/LightSumTypes.jl
index 1a275d8..c2f3503 100644
--- a/src/LightSumTypes.jl
+++ b/src/LightSumTypes.jl
@@ -211,15 +211,10 @@ function _get_variant_types(T_sum)
 end
 
 @generated function apply(f::F, args::Tuple) where {F}
-    
 
     args = fieldtypes(args)
     sumtype_args = [(i, T) for (i, T) in enumerate(args) if _is_sumtype_structurally(T)]
 
-    if isempty(sumtype_args)
-        return :(f(args...))
-    end
-
     final_args = Any[:(args[$i]) for i in 1:length(args)]
     for (idx, T) in sumtype_args
         final_args[idx] = Symbol("v_", idx)