Add apply function for type stable calls of functions

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)
 
@@ -194,6 +194,55 @@ 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)
+    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)]
+
+    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