[sc-36663] Lifts HTML Identifier module from schmods

src/DOM/Erumu/Identifier.purs

@@ -0,0 +1,61 @@
+module DOM.Erumu.Identifier
+  ( Identifier
+  , base
+  , str
+  , id_
+  , pred
+  , succ
+  ) where
+
+import Prelude
+
+import Data.Maybe (Maybe(Just))
+import Data.Enum as Enum
+
+import DOM.Erumu.HTML as HTML
+import DOM.Erumu.Types (Prop)
+
+type Sequence = Int
+
+data Identifier = Identifier Sequence String
+
+base :: String -> Identifier
+base = Identifier 0
+
+id_ :: forall msg. Identifier -> Prop msg
+id_ = HTML.id_ <<< str
+
+str :: Identifier -> String
+str (Identifier s b) = b <> show s
+
+-- Strictly speaking, succ could overflow (or pred could underflow) the
+-- 32-bit integer range. However, because we do not expose operations to
+-- either
+--
+--    a) construct Identifier sequence values with numbers other than 0
+--  or
+--    b) add (or substruct) any value greater than 1 from the sequence
+--
+-- in order to actually achieve the feat of overflowing succ would require
+-- 2^31 (2,147,483,648) operations to be performed. If you are here because
+-- you have managed to do this and have gotten bit by overflow, then I offer
+-- both congratulations and sincere condolences.
+--
+
+pred :: Identifier -> Identifier
+pred (Identifier s b) = Identifier (s - 1) b
+
+succ :: Identifier -> Identifier
+succ (Identifier s b) = Identifier (s + 1) b
+
+instance Eq Identifier where
+  eq (Identifier s1 b1) (Identifier s2 b2) = s1 == s2 && b1 == b2
+
+instance Ord Identifier where
+  compare (Identifier s1 b1) (Identifier s2 b2) =
+    compare b1 b2 <> compare s1 s2
+
+instance Enum.Enum Identifier where
+  pred = Just <<< pred
+  succ = Just <<< succ
+