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diff --git a/src/Numeric/InfInt.hs b/src/Numeric/InfInt.hs
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+{-|
+Module      : Numeric.InfInt
+Copyright   : (c) UU, 2019
+License     : MIT
+Maintainer  : Tom Smeding
+Stability   : experimental
+Portability : POSIX, macOS, Windows
+-}
+module Numeric.InfInt where
+
+
+-- | The integers with ±∞ added. This is not a full ring ('∞ + -∞' is
+-- undefined, for instance), but it works well enough.
+data InfInt = MInfinity | Finite Int | Infinity
+  deriving (Show, Eq, Ord)
+
+instance Num InfInt where
+    Infinity  + MInfinity = undefined
+    Infinity  + _         = Infinity
+    MInfinity + Infinity  = undefined
+    MInfinity + _         = MInfinity
+    Finite n  + Finite m  = Finite (n + m)
+    Finite _  + Infinity  = Infinity
+    Finite _  + MInfinity = MInfinity
+
+    Finite n  * Finite m  = Finite (n * m)
+    Finite 0  * _         = undefined
+    Finite n  * Infinity  = if n < 0 then MInfinity else Infinity
+    Finite n  * MInfinity = if n < 0 then Infinity else MInfinity
+    Infinity  * Finite m  = Finite m * Infinity
+    Infinity  * Infinity  = Infinity
+    Infinity  * MInfinity = MInfinity
+    MInfinity * Finite m  = Finite m * MInfinity
+    MInfinity * Infinity  = MInfinity
+    MInfinity * MInfinity = Infinity
+
+    abs (Finite n) = Finite (abs n)
+    abs _          = Infinity
+
+    signum (Finite n) = Finite (signum n)
+    signum MInfinity  = (-1)
+    signum Infinity   = 1
+
+    fromInteger n = Finite (fromInteger n)
+
+    negate (Finite n) = Finite (-n)
+    negate Infinity = MInfinity
+    negate MInfinity = Infinity
+
+
+-- | If the number is finite, return the finite component.
+toFinite :: InfInt -> Maybe Int
+toFinite (Finite n) = Just n
+toFinite Infinity   = Nothing
+toFinite MInfinity  = Nothing
+
+-- | @isFinite = isJust . toFinite@
+isFinite :: InfInt -> Bool
+isFinite (Finite _) = True
+isFinite _          = False