DROP: add some examples of Regex uses, that will be removed from production code.

drop/regex-and-validation-examples.purs

@@ -0,0 +1,44 @@
+import Data.String.Regex as R
+import Data.String.Regex.Flags as RF
+import Data.Validation.Semigroup (V, andThen, invalid, toEither)
+
+andThenDrop :: forall errors a b. V errors a -> V errors b -> V errors b
+-- andThenDrop f1 f2 = f1 !> (\ _ -> f2)
+
+infixl 8 andThen as !>
+-- infixl 8 andThenDrop as !<
+
+name_format :: String
+name_format = "[a-zA-Z]+"
+protocol_format :: String
+protocol_format = "^(tcp|udp|sctp)$"
+hostname_format :: String
+hostname_format = "^(([a-zA-Z0-9]|[a-zA-Z0-9][a-zA-Z0-9-]*[a-zA-Z0-9]).)*([A-Za-z0-9]|[A-Za-z0-9][A-Za-z0-9-]*[A-Za-z0-9])[.]?$"
+-- Basic tools for validation.
+
+lengthIsBetween :: Attribute -> Int -> Int -> String -> V Errors String
+lengthIsBetween field minlen maxlen value
+  = if valid_condition
+      then pure value
+      else invalid [ Tuple field error_message ]
+  where
+    actual_len = S.length value
+    valid_condition = actual_len >= minlen && actual_len <= maxlen
+    error_message = "acceptable length [" <> show minlen <> "-" <> show maxlen <> "]"
+
+-- | `matches` is a simple format verification based on regex parsing.
+-- | `verify_regex` is a handler to use `matches` with a string regex format.
+-- |
+-- | ```
+-- | verify_regex Name name_format name
+-- | ```
+matches :: Attribute -> String -> R.Regex -> V Errors String
+matches field value regex
+  | R.test regex value = pure value
+  | otherwise          = invalid [Tuple field "unacceptable format"]
+
+verify_regex :: Attribute -> String -> String -> V Errors String
+verify_regex field restr value
+  = case R.regex restr RF.unicode of
+      Left error_string -> invalid [Tuple field $ "error in regex: " <> error_string]
+      Right regex       -> matches field value regex