+fun isIdent ch = Char.isLower ch orelse Char.isDigit ch
+
+fun validHost s =
+ size s > 0 andalso size s < 20
+ andalso CharVector.all (fn ch => isIdent ch orelse ch = #"-") s
+
+fun validDomain s =
+ size s > 0 andalso size s < 100
+ andalso List.all validHost (String.fields (fn ch => ch = #".") s)
+
+val _ = Env.type_one "host"
+ Env.string
+ validHost
+
+val _ = Env.type_one "domain"
+ Env.string
+ validDomain
+