diff --git a/ml-proto/host/parser.mly b/ml-proto/host/parser.mly
index e8f4f239ce..814b15fc5e 100644
--- a/ml-proto/host/parser.mly
+++ b/ml-proto/host/parser.mly
@@ -200,9 +200,9 @@ expr1 :
   | BINARY expr expr { fun c -> binary ($1, $2 c, $3 c) }
   | COMPARE expr expr { fun c -> compare ($1, $2 c, $3 c) }
   | CONVERT expr { fun c -> convert ($1, $2 c) }
-  | PAGE_SIZE { fun c -> page_size }
-  | MEMORY_SIZE { fun c -> memory_size }
-  | RESIZE_MEMORY expr { fun c -> resize_memory ($2 c) }
+  | PAGE_SIZE { fun c -> host (PageSize, []) }
+  | MEMORY_SIZE { fun c -> host (MemorySize, []) }
+  | RESIZE_MEMORY expr { fun c -> host (ResizeMemory, [$2 c]) }
 ;
 expr_opt :
   | /* empty */ { fun c -> None }
diff --git a/ml-proto/spec/ast.ml b/ml-proto/spec/ast.ml
index 0bd14c35e6..425feff10e 100644
--- a/ml-proto/spec/ast.ml
+++ b/ml-proto/spec/ast.ml
@@ -66,6 +66,11 @@ type cvt = (Int32Op.cvt, Int64Op.cvt, Float32Op.cvt, Float64Op.cvt) op
 type memop = {ty : Types.value_type; align : int option}
 type extop = {memop : memop; sz : Memory.mem_size; ext : Memory.extension}
 type wrapop = {memop : memop; sz : Memory.mem_size}
+type hostop =
+  | PageSize           (* inquire host-defined page size *)
+  | MemorySize         (* inquire current size of linear memory *)
+  | ResizeMemory       (* resize linear memory *)
+
 
 (* Expressions *)
 
@@ -95,9 +100,7 @@ and expr' =
   | Binary of binop * expr * expr                  (* binary arithmetic operator *)
   | Compare of relop * expr * expr                 (* arithmetic comparison *)
   | Convert of cvt * expr                          (* conversion *)
-  | PageSize                                       (* return host-defined page_size *)
-  | MemorySize                                     (* return current size of linear memory *)
-  | ResizeMemory of expr                           (* resize linear memory *)
+  | Host of hostop * expr list                     (* host interaction *)
 
 and case = case' Source.phrase
 and case' =
diff --git a/ml-proto/spec/check.ml b/ml-proto/spec/check.ml
index 92a1cad6f3..00ebc651ad 100644
--- a/ml-proto/spec/check.ml
+++ b/ml-proto/spec/check.ml
@@ -89,6 +89,12 @@ let type_cvt at = function
     | DemoteFloat64 -> error at "invalid conversion"
     ), Float64Type
 
+let type_hostop = function
+  | PageSize -> {ins = []; out = Some Int32Type}
+  | MemorySize -> {ins = []; out = Some Int32Type}
+  | ResizeMemory -> {ins = [Int32Type]; out = None}
+
+
 let type_func f =
   let {params; result; _} = f.it in
   {ins = List.map it params; out = Lib.Option.map it result}
@@ -136,7 +142,7 @@ let rec check_expr c et e =
     check_expr c' et e1
 
   | Break (x, eo) ->
-    check_expr_option c (label c x) eo e.at
+    check_expr_opt c (label c x) eo e.at
 
   | Switch (t, e1, cs, e2) ->
     require (t.it = Int32Type || t.it = Int64Type) t.at "invalid switch type";
@@ -208,25 +214,18 @@ let rec check_expr c et e =
     check_expr c (Some t1) e1;
     check_type (Some t) et e.at
 
-  | PageSize ->
-    check_has_memory c e.at;
-    check_type (Some Int32Type) et e.at
-
-  | MemorySize ->
+  | Host (hostop, es) ->
     check_has_memory c e.at;
-    check_type (Some Int32Type) et e.at
-
-  | ResizeMemory e ->
-    check_has_memory c e.at;
-    check_expr c (Some Int32Type) e;
-    check_type None et e.at
+    let {ins; out} = type_hostop hostop in
+    check_exprs c ins es;
+    check_type out et e.at
 
 and check_exprs c ts es =
   let ets = List.map (fun x -> Some x) ts in
   try List.iter2 (check_expr c) ets es
   with Invalid_argument _ -> error (Source.ats es) "arity mismatch"
 
-and check_expr_option c et eo at =
+and check_expr_opt c et eo at =
   match eo with
   | Some e -> check_expr c et e
   | None -> check_type None et at
@@ -253,7 +252,7 @@ and check_store c et memop e1 e2 at =
   check_type (Some memop.ty) et at
 
 and check_has_memory c at =
-  require c.has_memory at "memory ops require a memory section";
+  require c.has_memory at "memory operators require a memory section"
 
 and check_align align at =
   Lib.Option.app (fun a ->
diff --git a/ml-proto/spec/eval.ml b/ml-proto/spec/eval.ml
index a55ebcae51..16c60040aa 100644
--- a/ml-proto/spec/eval.ml
+++ b/ml-proto/spec/eval.ml
@@ -143,7 +143,7 @@ let rec eval_expr (c : config) (e : expr) =
     (try eval_expr c' e1 with L.Label vo -> vo)
 
   | Break (x, eo) ->
-    raise (label c x (eval_expr_option c eo))
+    raise (label c x (eval_expr_opt c eo))
 
   | Switch (_t, e1, cs, e2) ->
     let vo = some (eval_expr c e1) e1.at in
@@ -189,7 +189,8 @@ let rec eval_expr (c : config) (e : expr) =
   | LoadExtend ({memop = {ty; align = _}; sz; ext}, e1) ->
     let mem = some_memory c e.at in
     let v1 = mem_size (eval_expr c e1) e1.at in
-    (try Some (Memory.load_extend mem v1 sz ext ty) with exn -> memory_error e.at exn)
+    (try Some (Memory.load_extend mem v1 sz ext ty)
+      with exn -> memory_error e.at exn)
 
   | StoreWrap ({memop = {ty; align = _}; sz}, e1, e2) ->
     let mem = some_memory c e.at in
@@ -233,25 +234,12 @@ let rec eval_expr (c : config) (e : expr) =
       | Arithmetic.TypeError (_, v, t) -> type_error e1.at v t
       | exn -> numerics_error e.at exn)
 
-  | PageSize ->
-    Some (Int32 (Int64.to_int32 c.instance.host.page_size))
-
-  | MemorySize ->
+  | Host (hostop, es) ->
+    let vs = List.map (eval_expr c) es in
     let mem = some_memory c e.at in
-    let i64 = Memory.size mem in
-    assert (i64 < Int64.of_int32 (Int32.max_int));
-    Some (Int32 (Int64.to_int32 i64))
+    eval_hostop c.instance.host mem hostop vs e.at
 
-  | ResizeMemory e ->
-    let mem = some_memory c e.at in
-    let sz = mem_size (eval_expr c e) e.at in
-    if (Int64.rem sz c.instance.host.page_size) <> 0L then
-      error e.at "runtime: resize_memory operand not multiple of page_size";
-    Memory.resize mem sz;
-    None
-
-and eval_expr_option c eo =
-  match eo with
+and eval_expr_opt c = function
   | Some e -> eval_expr c e
   | None -> None
 
@@ -273,6 +261,28 @@ and eval_func (m : instance) (f : func) (evs : value list) =
   eval_expr c f.it.body
 
 
+(* Host operators *)
+
+and eval_hostop host mem hostop vs at =
+  match hostop, vs with
+  | PageSize, [] ->
+    Some (Int32 (Int64.to_int32 host.page_size))
+
+  | MemorySize, [] ->
+    assert (Memory.size mem < Int64.of_int32 Int32.max_int);
+    Some (Int32 (Int64.to_int32 (Memory.size mem)))
+
+  | ResizeMemory, [v] ->
+    let sz = mem_size v at in
+    if Int64.rem sz host.page_size <> 0L then
+      error at "runtime: resize_memory operand not multiple of page_size";
+    Memory.resize mem sz;
+    None
+
+  | _, _ ->
+    error at "runtime: invalid invocation of host operator"
+
+
 (* Modules *)
 
 let init_memory {it = {initial; segments; _}} =
@@ -304,5 +314,5 @@ let host_eval e =
   let f = {params = []; result = None; locals = []; body = e} @@ no_region in
   let exports = ExportMap.singleton "eval" f in
   let host = {page_size = 1L} in
-  let m = {imports = []; exports; tables = []; funcs = [f]; memory = None; host} in
-  eval_func m f []
+  let m = {imports = []; exports; tables = []; funcs = [f]; memory = None; host}
+  in eval_func m f []
diff --git a/ml-proto/spec/memory.ml b/ml-proto/spec/memory.ml
index 336a36ffd5..9abd21b835 100644
--- a/ml-proto/spec/memory.ml
+++ b/ml-proto/spec/memory.ml
@@ -22,6 +22,7 @@ exception Type
 exception Bounds
 exception Address
 
+
 (*
  * These limitations should be considered part of the host environment and not
  * part of the spec defined by this file.
diff --git a/ml-proto/spec/memory.mli b/ml-proto/spec/memory.mli
index 2b3320a00d..e830307fcc 100644
--- a/ml-proto/spec/memory.mli
+++ b/ml-proto/spec/memory.mli
@@ -22,5 +22,6 @@ val size : memory -> size
 val resize : memory -> size -> unit
 val load : memory -> address -> value_type -> value
 val store : memory -> address -> value -> unit
-val load_extend : memory -> address -> mem_size -> extension -> value_type -> value
+val load_extend :
+  memory -> address -> mem_size -> extension -> value_type -> value
 val store_wrap : memory -> address -> mem_size -> value -> unit
diff --git a/ml-proto/spec/sugar.ml b/ml-proto/spec/sugar.ml
index 134bb4dd1b..7d4b8946ba 100644
--- a/ml-proto/spec/sugar.ml
+++ b/ml-proto/spec/sugar.ml
@@ -85,14 +85,8 @@ let compare (relop, e1, e2) =
 let convert (cvt, e) =
   Convert (cvt, e)
 
-let page_size =
-  PageSize
-
-let memory_size =
-  MemorySize
-
-let resize_memory e =
-  ResizeMemory e
+let host (hostop, es) =
+  Host (hostop, es)
 
 
 let case (c, br) =
diff --git a/ml-proto/spec/sugar.mli b/ml-proto/spec/sugar.mli
index 2cb3cdf3ca..6e82ee8840 100644
--- a/ml-proto/spec/sugar.mli
+++ b/ml-proto/spec/sugar.mli
@@ -25,9 +25,7 @@ val unary : unop * expr -> expr'
 val binary : binop * expr * expr -> expr'
 val compare : relop * expr * expr -> expr'
 val convert : cvt * expr -> expr'
-val page_size : expr'
-val memory_size : expr'
-val resize_memory : expr -> expr'
+val host : hostop * expr list -> expr'
 
 val case : literal * (expr list * bool) option -> case'