Refactor: encapsulate variadic functions in typechecking

docs/exposing_new_functions.mld

@@ -130,8 +130,15 @@ For example, the following line defines the signature [add(real, matrix) => matr
 Functions such as the ODE integrators or [reduce_sum], which take in user-functions and a variable-length
 list of arguments, are {b NOT} added to this list.
 
-These are instead treated as special cases in the [Typechecker] module in the frontend folder. It
-it best to consult an existing example of how these are done before proceeding.
+"Nice" variadic functions are added to the hashtable [Stan_math_signatures.stan_math_variadic_signatures].
+This is probably sufficient for most variadic functions, e.g. all the ODE solvers and DAE solvers are done
+via this method.
+[reduce_sum] is not "nice", since it is both variadic and {e polymorphic}, requiring certain arguments to have the same
+(but {e not predetermined}) type. Therefore, [reduce_sum] is treated as special case in the [Typechecker]
+module in the frontend folder.
+
+Note that higher-order functions also usually require changes to the C++ code generation to work properly.
+It is best to consult an existing example of how these are done before proceeding.
 
 {1 Testing}
 

src/analysis_and_optimization/Memory_patterns.ml

@@ -113,9 +113,8 @@ let query_stan_math_mem_pattern_support (name : string)
     (args : (UnsizedType.autodifftype * UnsizedType.t) list) =
   let open Stan_math_signatures in
   match name with
+  | x when is_stan_math_variadic_function_name x -> false
   | x when is_reduce_sum_fn x -> false
-  | x when is_variadic_ode_fn x -> false
-  | x when is_variadic_dae_fn x -> false
   | _ ->
       let name =
         string_operator_to_stan_math_fns (Utils.stdlib_distribution_name name)

src/frontend/Semantic_error.ml

@@ -26,7 +26,7 @@ module TypeError = struct
         * UnsizedType.t list
         * (UnsizedType.autodifftype * UnsizedType.t) list
         * SignatureMismatch.function_mismatch
-    | IllTypedVariadicDE of
+    | IllTypedVariadic of
         string
         * UnsizedType.t list
         * (UnsizedType.autodifftype * UnsizedType.t) list
@@ -131,7 +131,7 @@ module TypeError = struct
     | IllTypedReduceSumGeneric (name, arg_tys, expected_args, error) ->
         SignatureMismatch.pp_signature_mismatch ppf
           (name, arg_tys, ([((ReturnType UReal, expected_args), error)], false))
-    | IllTypedVariadicDE (name, arg_tys, args, error, return_type) ->
+    | IllTypedVariadic (name, arg_tys, args, error, return_type) ->
         SignatureMismatch.pp_signature_mismatch ppf
           ( name
           , arg_tys
@@ -550,25 +550,8 @@ let illtyped_reduce_sum_generic loc name arg_tys expected_args error =
     , TypeError.IllTypedReduceSumGeneric (name, arg_tys, expected_args, error)
     )
 
-let illtyped_variadic_ode loc name arg_tys args error =
-  TypeError
-    ( loc
-    , TypeError.IllTypedVariadicDE
-        ( name
-        , arg_tys
-        , args
-        , error
-        , Stan_math_signatures.variadic_ode_fun_return_type ) )
-
-let illtyped_variadic_dae loc name arg_tys args error =
-  TypeError
-    ( loc
-    , TypeError.IllTypedVariadicDE
-        ( name
-        , arg_tys
-        , args
-        , error
-        , Stan_math_signatures.variadic_dae_fun_return_type ) )
+let illtyped_variadic loc name arg_tys args fn_rt error =
+  TypeError (loc, TypeError.IllTypedVariadic (name, arg_tys, args, error, fn_rt))
 
 let ambiguous_function_promotion loc name arg_tys signatures =
   TypeError

src/frontend/Semantic_error.mli

@@ -58,14 +58,6 @@ val illtyped_reduce_sum_generic :
   -> SignatureMismatch.function_mismatch
   -> t
 
-val illtyped_variadic_ode :
-     Location_span.t
-  -> string
-  -> UnsizedType.t list
-  -> (UnsizedType.autodifftype * UnsizedType.t) list
-  -> SignatureMismatch.function_mismatch
-  -> t
-
 val ambiguous_function_promotion :
      Location_span.t
   -> string
@@ -74,11 +66,12 @@ val ambiguous_function_promotion :
      list
   -> t
 
-val illtyped_variadic_dae :
+val illtyped_variadic :
      Location_span.t
   -> string
   -> UnsizedType.t list
   -> (UnsizedType.autodifftype * UnsizedType.t) list
+  -> UnsizedType.t
   -> SignatureMismatch.function_mismatch
   -> t
 

src/frontend/SignatureMismatch.ml

@@ -248,7 +248,7 @@ let matching_function env name args =
 let matching_stanlib_function =
   matching_function Environment.stan_math_environment
 
-let check_variadic_args allow_lpdf mandatory_arg_tys mandatory_fun_arg_tys
+let check_variadic_args ~allow_lpdf mandatory_arg_tys mandatory_fun_arg_tys
     fun_return args =
   let minimal_func_type =
     UnsizedType.UFun (mandatory_fun_arg_tys, ReturnType fun_return, FnPlain, AoS)

src/frontend/SignatureMismatch.mli

@@ -62,7 +62,7 @@ val matching_stanlib_function :
 *)
 
 val check_variadic_args :
-     bool
+     allow_lpdf:bool
   -> (UnsizedType.autodifftype * UnsizedType.t) list
   -> (UnsizedType.autodifftype * UnsizedType.t) list
   -> UnsizedType.t

src/frontend/Typechecker.ml

@@ -122,8 +122,7 @@ let verify_name_fresh_udf loc tenv name =
     (* variadic functions are currently not in math sigs and aren't
        overloadable due to their separate typechecking *)
     Stan_math_signatures.is_reduce_sum_fn name
-    || Stan_math_signatures.is_variadic_ode_fn name
-    || Stan_math_signatures.is_variadic_dae_fn name
+    || Stan_math_signatures.is_stan_math_variadic_function_name name
   then Semantic_error.ident_is_stanmath_name loc name |> error
   else if Utils.is_unnormalized_distribution name then
     Semantic_error.udf_is_unnormalized_fn loc name |> error
@@ -191,14 +190,13 @@ let match_to_rt_option = function
   | _ -> None
 
 let stan_math_return_type name arg_tys =
-  match name with
-  | x when Stan_math_signatures.is_reduce_sum_fn x ->
+  match
+    Hashtbl.find Stan_math_signatures.stan_math_variadic_signatures name
+  with
+  | Some {return_type; _} -> Some (UnsizedType.ReturnType return_type)
+  | None when Stan_math_signatures.is_reduce_sum_fn name ->
       Some (UnsizedType.ReturnType UReal)
-  | x when Stan_math_signatures.is_variadic_ode_fn x ->
-      Some (UnsizedType.ReturnType (UArray UVector))
-  | x when Stan_math_signatures.is_variadic_dae_fn x ->
-      Some (UnsizedType.ReturnType (UArray UVector))
-  | _ ->
+  | None ->
       SignatureMismatch.matching_stanlib_function name arg_tys
       |> match_to_rt_option
 
@@ -571,30 +569,25 @@ let make_function_variable cf loc id = function
 
 let rec check_fn ~is_cond_dist loc cf tenv id (tes : Ast.typed_expression list)
     =
-  if Stan_math_signatures.is_reduce_sum_fn id.name then
+  if Stan_math_signatures.is_stan_math_variadic_function_name id.name then
+    check_variadic ~is_cond_dist loc cf tenv id tes
+  else if Stan_math_signatures.is_reduce_sum_fn id.name then
     check_reduce_sum ~is_cond_dist loc cf tenv id tes
-  else if Stan_math_signatures.is_variadic_ode_fn id.name then
-    check_variadic_ode ~is_cond_dist loc cf tenv id tes
-  else if Stan_math_signatures.is_variadic_dae_fn id.name then
-    check_variadic_dae ~is_cond_dist loc cf tenv id tes
   else check_normal_fn ~is_cond_dist loc tenv id tes
 
+(** Reduce sum is a special case, even compared to the other
+    variadic functions, because it is polymorphic in the type of the
+    first argument. The first, fourth, and fifth arguments must agree,
+    which is too complicated to be captured declaratively. *)
 and check_reduce_sum ~is_cond_dist loc cf tenv id tes =
   let basic_mismatch () =
     let mandatory_args =
       UnsizedType.[(AutoDiffable, UArray UReal); (AutoDiffable, UInt)] in
     let mandatory_fun_args =
       UnsizedType.
         [(AutoDiffable, UArray UReal); (DataOnly, UInt); (DataOnly, UInt)] in
-    SignatureMismatch.check_variadic_args true mandatory_args mandatory_fun_args
-      UReal (get_arg_types tes) in
-  let fail () =
-    let expected_args, err =
-      basic_mismatch () |> Result.error |> Option.value_exn in
-    Semantic_error.illtyped_reduce_sum_generic loc id.name
-      (List.map ~f:type_of_expr_typed tes)
-      expected_args err
-    |> error in
+    SignatureMismatch.check_variadic_args ~allow_lpdf:true mandatory_args
+      mandatory_fun_args UReal (get_arg_types tes) in
   let matching remaining_es fn =
     match fn with
     | Env.
@@ -611,7 +604,7 @@ and check_reduce_sum ~is_cond_dist loc cf tenv id tes =
         let arg_types =
           (calculate_autodifftype cf Functions ftype, ftype)
           :: get_arg_types remaining_es in
-        SignatureMismatch.check_variadic_args true mandatory_args
+        SignatureMismatch.check_variadic_args ~allow_lpdf:true mandatory_args
           mandatory_fun_args UReal arg_types
     | _ -> basic_mismatch () in
   match tes with
@@ -633,81 +626,25 @@ and check_reduce_sum ~is_cond_dist loc cf tenv id tes =
           (List.map ~f:type_of_expr_typed tes)
           expected_args err
         |> error )
-  | _ -> fail ()
-
-and check_variadic_ode ~is_cond_dist loc cf tenv id tes =
-  let optional_tol_mandatory_args =
-    if Stan_math_signatures.variadic_ode_adjoint_fn = id.name then
-      Stan_math_signatures.variadic_ode_adjoint_ctl_tol_arg_types
-    else if Stan_math_signatures.is_variadic_ode_nonadjoint_tol_fn id.name then
-      Stan_math_signatures.variadic_ode_tol_arg_types
-    else [] in
-  let mandatory_arg_types =
-    Stan_math_signatures.variadic_ode_mandatory_arg_types
-    @ optional_tol_mandatory_args in
-  let fail () =
-    let expected_args, err =
-      SignatureMismatch.check_variadic_args false mandatory_arg_types
-        Stan_math_signatures.variadic_ode_mandatory_fun_args
-        Stan_math_signatures.variadic_ode_fun_return_type (get_arg_types tes)
-      |> Result.error |> Option.value_exn in
-    Semantic_error.illtyped_variadic_ode loc id.name
-      (List.map ~f:type_of_expr_typed tes)
-      expected_args err
-    |> error in
-  let matching remaining_es Env.{type_= ftype; _} =
-    let arg_types =
-      (calculate_autodifftype cf Functions ftype, ftype)
-      :: get_arg_types remaining_es in
-    SignatureMismatch.check_variadic_args false mandatory_arg_types
-      Stan_math_signatures.variadic_ode_mandatory_fun_args
-      Stan_math_signatures.variadic_ode_fun_return_type arg_types in
-  match tes with
-  | {expr= Variable fname; _} :: remaining_es -> (
-    match find_matching_first_order_fn tenv (matching remaining_es) fname with
-    | SignatureMismatch.UniqueMatch (ftype, promotions) ->
-        let tes = make_function_variable cf loc fname ftype :: remaining_es in
-        mk_typed_expression
-          ~expr:
-            (mk_fun_app ~is_cond_dist
-               (StanLib FnPlain, id, Promotion.promote_list tes promotions) )
-          ~ad_level:(expr_ad_lub tes)
-          ~type_:Stan_math_signatures.variadic_ode_return_type ~loc
-    | AmbiguousMatch ps ->
-        Semantic_error.ambiguous_function_promotion loc fname.name None ps
-        |> error
-    | SignatureErrors (expected_args, err) ->
-        Semantic_error.illtyped_variadic_ode loc id.name
-          (List.map ~f:type_of_expr_typed tes)
-          expected_args err
-        |> error )
-  | _ -> fail ()
-
-and check_variadic_dae ~is_cond_dist loc cf tenv id tes =
-  let optional_tol_mandatory_args =
-    if Stan_math_signatures.is_variadic_dae_tol_fn id.name then
-      Stan_math_signatures.variadic_dae_tol_arg_types
-    else [] in
-  let mandatory_arg_types =
-    Stan_math_signatures.variadic_dae_mandatory_arg_types
-    @ optional_tol_mandatory_args in
-  let fail () =
-    let expected_args, err =
-      SignatureMismatch.check_variadic_args false mandatory_arg_types
-        Stan_math_signatures.variadic_dae_mandatory_fun_args
-        Stan_math_signatures.variadic_dae_fun_return_type (get_arg_types tes)
-      |> Result.error |> Option.value_exn in
-    Semantic_error.illtyped_variadic_dae loc id.name
-      (List.map ~f:type_of_expr_typed tes)
-      expected_args err
-    |> error in
+  | _ ->
+      let expected_args, err =
+        basic_mismatch () |> Result.error |> Option.value_exn in
+      Semantic_error.illtyped_reduce_sum_generic loc id.name
+        (List.map ~f:type_of_expr_typed tes)
+        expected_args err
+      |> error
+
+and check_variadic ~is_cond_dist loc cf tenv id tes =
+  let Stan_math_signatures.
+        {control_args; required_fn_args; required_fn_rt; return_type} =
+    Hashtbl.find_exn Stan_math_signatures.stan_math_variadic_signatures id.name
+  in
   let matching remaining_es Env.{type_= ftype; _} =
     let arg_types =
       (calculate_autodifftype cf Functions ftype, ftype)
       :: get_arg_types remaining_es in
-    SignatureMismatch.check_variadic_args false mandatory_arg_types
-      Stan_math_signatures.variadic_dae_mandatory_fun_args
-      Stan_math_signatures.variadic_dae_fun_return_type arg_types in
+    SignatureMismatch.check_variadic_args ~allow_lpdf:false control_args
+      required_fn_args required_fn_rt arg_types in
   match tes with
   | {expr= Variable fname; _} :: remaining_es -> (
     match find_matching_first_order_fn tenv (matching remaining_es) fname with
@@ -717,17 +654,24 @@ and check_variadic_dae ~is_cond_dist loc cf tenv id tes =
           ~expr:
             (mk_fun_app ~is_cond_dist
                (StanLib FnPlain, id, Promotion.promote_list tes promotions) )
-          ~ad_level:(expr_ad_lub tes)
-          ~type_:Stan_math_signatures.variadic_dae_return_type ~loc
+          ~ad_level:(expr_ad_lub tes) ~type_:return_type ~loc
     | AmbiguousMatch ps ->
         Semantic_error.ambiguous_function_promotion loc fname.name None ps
         |> error
     | SignatureErrors (expected_args, err) ->
-        Semantic_error.illtyped_variadic_dae loc id.name
+        Semantic_error.illtyped_variadic loc id.name
           (List.map ~f:type_of_expr_typed tes)
-          expected_args err
+          expected_args required_fn_rt err
         |> error )
-  | _ -> fail ()
+  | _ ->
+      let expected_args, err =
+        SignatureMismatch.check_variadic_args ~allow_lpdf:false control_args
+          required_fn_args required_fn_rt (get_arg_types tes)
+        |> Result.error |> Option.value_exn in
+      Semantic_error.illtyped_variadic loc id.name
+        (List.map ~f:type_of_expr_typed tes)
+        expected_args required_fn_rt err
+      |> error
 
 and check_funapp loc cf tenv ~is_cond_dist id (es : Ast.typed_expression list) =
   let name_check =