diff --git a/Data/Real/Constructible.hs b/Data/Real/Constructible.hs
new file mode 100644
--- /dev/null
+++ b/Data/Real/Constructible.hs
@@ -0,0 +1,405 @@
+{-# LANGUAGE
+  DataKinds,
+  DeriveDataTypeable,
+  FlexibleInstances,
+  GADTs,
+  MultiParamTypeClasses,
+  TemplateHaskell,
+  TypeFamilies #-}
+
+{- |
+Module      :  Data.Real.Constructible
+Description :  Constructible real numbers
+Copyright   :  © Anders Kaseorg, 2013
+License     :  BSD-style
+
+Maintainer  :  Anders Kaseorg <andersk@mit.edu>
+Stability   :  experimental
+Portability :  Non-portable (GHC extensions)
+
+The constructible reals, 'Construct', are the subset of the real
+numbers that can be represented exactly using field operations
+(addition, subtraction, multiplication, division) and positive square
+roots.  They support exact computations, equality comparisons, and
+ordering.
+
+>>> [((1 + sqrt 5)/2)^n - ((1 - sqrt 5)/2)^n :: Construct | n <- [1..10]]
+[sqrt 5,sqrt 5,2*sqrt 5,3*sqrt 5,5*sqrt 5,8*sqrt 5,13*sqrt 5,21*sqrt 5,34*sqrt 5,55*sqrt 5]
+
+>>> let f (a, b, t, p) = ((a + b)/2, sqrt (a*b), t - p*((a - b)/2)^2, 2*p)
+>>> let (a, b, t, p) = f . f . f . f $ (1, 1/sqrt 2, 1/4, 1 :: Construct)
+>>> floor $ ((a + b)^2/(4*t))*10**40
+31415926535897932384626433832795028841971
+
+>>> let qf (p, q) = ((p + sqrt (p^2 - 4*q))/2, (p - sqrt (p^2 - 4*q))/2 :: Construct)
+>>> let [(v, w), (x, _), (y, _), (z, _)] = map qf [(-1, -4), (v, -1), (w, -1), (x, y)]
+>>> z/2
+-1/16 + 1/16*sqrt 17 + 1/8*sqrt (17/2 - 1/2*sqrt 17) + 1/4*sqrt (17/4 + 3/4*sqrt 17 - (3/4 + 1/4*sqrt 17)*sqrt (17/2 - 1/2*sqrt 17))
+
+Constructible complex numbers may be built from constructible reals
+using 'Complex' from the complex-generic library.
+
+>>> (z/2 :+ sqrt (1 - (z/2)^2))^17
+1 :+ 0
+-}
+
+module Data.Real.Constructible (
+  Construct,
+  deconstruct,
+  fromConstruct,
+  ConstructException (..)) where
+
+import Control.Applicative ((<$>), (<*>), (<|>), empty)
+import Control.Exception (Exception, ArithException (..), throw)
+import Data.Complex.Generic (Complex (..))
+import Data.Complex.Generic.TH (deriveComplexF)
+import Data.Ratio ((%), numerator, denominator)
+import Data.Typeable (Typeable)
+import Math.NumberTheory.Powers.Squares (exactSquareRoot)
+import Numeric.Search.Integer (search)
+import Text.Read (Lexeme (..), Read (..), lexP, parens, prec, readListPrecDefault, step)
+import Text.Read.Lex (numberToInteger)
+
+data FieldShape = QShape | SqrtShape !FieldShape deriving Show
+
+data Field k where
+  Q :: Field QShape
+  Sqrt :: !(Field k) -> !(Elt k) -> Field (SqrtShape k)
+
+instance Show (Field k) where
+  showsPrec _ Q = showString "Q"
+  showsPrec d (Sqrt k r) =
+    showParen (d > 9) $ showsPrec 9 k . showString "[sqrt " . showsPrecK k 10 r . showString "]"
+
+type family Elt (k :: FieldShape)
+type instance Elt QShape = Rational
+data SqrtElt k = SqrtZero | SqrtElt !(Elt k) !(Elt k)
+type instance Elt (SqrtShape k) = SqrtElt k
+
+sqrtElt :: Field k -> Elt k -> Elt k -> SqrtElt k
+sqrtElt k a b | isZeroK k a && isZeroK k b = SqrtZero
+              | otherwise = SqrtElt a b
+
+sqrtLift :: Field k -> Elt k -> SqrtElt k
+sqrtLift k a = sqrtElt k a (zeroK k)
+
+addK :: Field k -> Elt k -> Elt k -> Elt k
+addK Q a b = a + b
+addK Sqrt{} SqrtZero a = a
+addK Sqrt{} a SqrtZero = a
+addK (Sqrt k _) (SqrtElt a b) (SqrtElt c d) = sqrtElt k (addK k a c) (addK k b d)
+
+mulK :: Field k -> Elt k -> Elt k -> Elt k
+mulK Q a b = a * b
+mulK Sqrt{} SqrtZero _ = SqrtZero
+mulK Sqrt{} _ SqrtZero = SqrtZero
+mulK (Sqrt k r) (SqrtElt a b) (SqrtElt c d) =
+  SqrtElt (addK k (mulK k a c) (mulK k r (mulK k b d))) (addK k (mulK k a d) (mulK k b c))
+
+subK :: Field k -> Elt k -> Elt k -> Elt k
+subK Q a b = a - b
+subK k@Sqrt{} SqrtZero a = negateK k a
+subK Sqrt{} a SqrtZero = a
+subK (Sqrt k _) (SqrtElt a b) (SqrtElt c d) = sqrtElt k (subK k a c) (subK k b d)
+
+negateK :: Field k -> Elt k -> Elt k
+negateK Q a = negate a
+negateK Sqrt{} SqrtZero = SqrtZero
+negateK (Sqrt k _) (SqrtElt a b) = SqrtElt (negateK k a) (negateK k b)
+
+absK :: Field k -> Elt k -> Elt k
+absK Q a = abs a
+absK k a = if sgnK k a == LT then negateK k a else a
+
+signumK :: Field k -> Elt k -> Rational
+signumK Q a = signum a
+signumK k a = case sgnK k a of LT -> -1; EQ -> 0; GT -> 1
+
+divK :: Field k -> Elt k -> Elt k -> Elt k
+divK Q a b = a / b
+divK k a b = mulK k a (recipK k b)
+
+recipK :: Field k -> Elt k -> Elt k
+recipK Q a = recip a
+recipK Sqrt{} SqrtZero = throw DivideByZero
+recipK (Sqrt k r) (SqrtElt a b) =
+  let c = recipK k (subK k (mulK k a a) (mulK k r (mulK k b b)))
+  in SqrtElt (mulK k a c) (mulK k (negateK k b) c)
+
+eqK :: Field k -> Elt k -> Elt k -> Bool
+eqK Q a b = a == b
+eqK Sqrt{} SqrtZero SqrtZero = True
+eqK (Sqrt k _) (SqrtElt a b) (SqrtElt c d) = eqK k a c && eqK k b d
+eqK Sqrt{} SqrtZero SqrtElt{} = False
+eqK Sqrt{} SqrtElt{} SqrtZero = False
+
+isZeroK :: Field k -> Elt k -> Bool
+isZeroK Q a = a == 0
+isZeroK Sqrt{} SqrtZero = True
+isZeroK Sqrt{} SqrtElt{} = False
+
+compareK :: Field k -> Elt k -> Elt k -> Ordering
+compareK Q a b = compare a b
+compareK k a b = sgnK k (subK k a b)
+
+sgnK :: Field k -> Elt k -> Ordering
+sgnK Q a = compare a 0
+sgnK Sqrt{} SqrtZero = EQ
+sgnK (Sqrt k r) (SqrtElt a b) = case (sgnK k a, sgnK k b) of
+  (o, EQ) -> o
+  (EQ, o) -> o
+  (GT, GT) -> GT
+  (LT, LT) -> LT
+  (GT, LT) -> sgnK k (subK k (mulK k a a) (mulK k r (mulK k b b)))
+  (LT, GT) -> sgnK k (subK k (mulK k r (mulK k b b)) (mulK k a a))
+
+zeroK :: Field k -> Elt k
+zeroK Q = 0
+zeroK Sqrt{} = SqrtZero
+
+fromRationalK :: Field k -> Rational -> Elt k
+fromRationalK Q a = a
+fromRationalK (Sqrt k _) a = sqrtLift k (fromRationalK k a)
+
+sqrtK :: Field k -> Elt k -> Maybe (Elt k)
+sqrtK Q a = (%) <$> exactSquareRoot (numerator a) <*> exactSquareRoot (denominator a)
+sqrtK Sqrt{} SqrtZero = return SqrtZero
+sqrtK (Sqrt k r) (SqrtElt a b)
+  | isZeroK k b = sqrtLift k <$> sqrtK k a <|> SqrtElt (zeroK k) <$> sqrtK k (divK k a r)
+  | otherwise = do
+    n <- sqrtK k $ subK k (mulK k a a) (mulK k r (mulK k b b))
+    let half = fromRationalK k (1 % 2)
+        p = mulK k half (addK k a n)
+        q = mulK k half b
+        y1 = do
+          c <- sqrtK k p
+          return (SqrtElt c (divK k q c))
+        y2 = do
+          d <- sqrtK k $ divK k p r
+          return (SqrtElt (divK k q d) d)
+    y1 <|> y2
+
+negateS, sqrtS :: (Int -> ShowS) -> Int -> ShowS
+(-!), (+!), (*!), (/!) :: (Int -> ShowS) -> (Int -> ShowS) -> Int -> ShowS
+infixl 6 +!, -!
+infixl 7 *!, /!
+negateS s d = showParen (d > 6) $ showChar '-' . s 6
+(+!) s1 s2 d = showParen (d > 6) $ s1 6 . showString " + " . s2 7
+(-!) s1 s2 d = showParen (d > 6) $ s1 6 . showString " - " . s2 7
+(*!) s1 s2 d = showParen (d > 7) $ s1 7 . showChar '*' . s2 8
+(/!) s1 s2 d = showParen (d > 7) $ s1 7 . showChar '/' . s2 8
+sqrtS s d = showParen (d > 9) $ showString "sqrt " . s 10
+
+mulSqrtS :: Field k -> Elt k -> Elt k -> Int -> ShowS
+mulSqrtS k b r
+  | eqK k b (fromRationalK k 1) = sqrtS (flip (showsPrecK k) r)
+  | otherwise = flip (showsPrecK k) b *! sqrtS (flip (showsPrecK k) r)
+
+showsPrecK :: Field k -> Int -> Elt k -> ShowS
+showsPrecK Q d x
+  | q == 1 = showsPrec d p
+  | p < 0 = negateS (flip showsPrec (-p) /! flip showsPrec q) d
+  | otherwise = (flip showsPrec p /! flip showsPrec q) d
+  where
+    p = numerator x
+    q = denominator x
+showsPrecK Sqrt{} _ SqrtZero = showChar '0'
+showsPrecK (Sqrt k r) d (SqrtElt a b) = case sgnK k b of
+  EQ -> showsPrecK k d a
+  GT | isZeroK k a -> mulSqrtS k b r d
+     | otherwise -> (flip (showsPrecK k) a +! mulSqrtS k b r) d
+  LT | isZeroK k a -> negateS (mulSqrtS k (negateK k b) r) d
+     | otherwise -> (flip (showsPrecK k) a -! mulSqrtS k (negateK k b) r) d
+
+fromConstructK :: Floating a => Field k -> Elt k -> a
+fromConstructK Q = fromRational
+fromConstructK kr@(Sqrt k r) = er where
+  e = fromConstructK k
+  s = sqrt (e r)
+  er SqrtZero = 0
+  er x@(SqrtElt a b) = case (sgnK k a, sgnK k b) of
+    (_, EQ) -> e a
+    (EQ, _) -> e b*s
+    (GT, GT) -> x1
+    (LT, LT) -> x1
+    (GT, LT) -> x2
+    (LT, GT) -> x2
+    where
+      x1 = e a + e b*s
+      SqrtElt c d = recipK kr x
+      x2 = recip $ e c + e d*s
+
+-- |The type of constructible real numbers.
+data Construct where
+  C :: !(Field k) -> !(Elt k) -> Construct
+
+deconstructK :: Field k -> Elt k -> Either Rational (Construct, Construct, Construct)
+deconstructK Q a = Left a
+deconstructK Sqrt{} SqrtZero = Left 0
+deconstructK (Sqrt k r) (SqrtElt a b)
+  | isZeroK k b = deconstructK k a
+  | otherwise = Right (C k a, C k b, C k r)
+
+{- |
+Deconstruct a constructible number as either a 'Rational', or a triple
+@(a, b, r)@ of simpler constructible numbers representing @a + b*sqrt
+r@ (with @b /= 0@ and @r > 0@).  Recursively calling 'deconstruct' on
+all triples will yield a finite tree that terminates in 'Rational'
+leaves.  Note that two constructible numbers that compare as equal may
+deconstruct in different ways.
+-}
+deconstruct :: Construct -> Either Rational (Construct, Construct, Construct)
+deconstruct (C k a) = deconstructK k a
+
+data JoinK k1 k2 where
+  JoinK :: !(Field k) -> (Elt k1 -> Elt k) -> (Elt k2 -> Elt k) -> JoinK k1 k2
+
+joinK :: Field k1 -> Field k2 -> JoinK k1 k2
+joinK Q k = JoinK k (fromRationalK k) id
+joinK k Q = JoinK k id (fromRationalK k)
+joinK k1 (Sqrt k2 r) = case joinK k1 k2 of
+  JoinK k f1 f2 -> let r' = f2 r in case sqrtK k r' of
+    Nothing ->
+      let f2' SqrtZero = SqrtZero
+          f2' (SqrtElt a b) = SqrtElt (f2 a) (f2 b)
+      in JoinK (Sqrt k r') (sqrtLift k . f1) f2'
+    Just s ->
+      let f2' SqrtZero = zeroK k
+          f2' (SqrtElt a b) = addK k (f2 a) (mulK k (f2 b) s)
+      in JoinK k f1 f2'
+
+instance Show Construct where
+  showsPrec d (C k a) = showsPrecK k d a
+
+instance Read Construct where
+  readPrec =
+    parens $
+    pNum <|>
+    prec 6 (pNegate <|> (step readPrec >>= pAddSub)) <|>
+    prec 7 (step readPrec >>= pMulDiv) <|>
+    prec 10 pSqrt
+    where
+      pNum = do {Number n <- lexP; maybe empty (return . fromInteger) (numberToInteger n)}
+      pNegate = do {Symbol "-" <- lexP; a <- negate <$> step readPrec; return a <|> pAddSub a}
+      pAddSub a = do {Symbol "+" <- lexP; b <- (a +) <$> step readPrec; return b <|> pAddSub b} <|>
+                  do {Symbol "-" <- lexP; b <- (a -) <$> step readPrec; return b <|> pAddSub b}
+      pMulDiv a = do {Symbol "*" <- lexP; b <- (a *) <$> step readPrec; return b <|> pMulDiv b} <|>
+                  do {Symbol "/" <- lexP; b <- (a /) <$> step readPrec; return b <|> pMulDiv b}
+      pSqrt = do {Ident "sqrt" <- lexP; a <- step readPrec; return (sqrt a)}
+  readListPrec = readListPrecDefault
+
+instance Eq Construct where
+  C k1 a1 == C k2 a2 = case joinK k1 k2 of JoinK k f1 f2 -> eqK k (f1 a1) (f2 a2)
+
+instance Ord Construct where
+  compare (C k1 a1) (C k2 a2) = case joinK k1 k2 of JoinK k f1 f2 -> compareK k (f1 a1) (f2 a2)
+
+instance Num Construct where
+  C k1 a1 + C k2 a2 = case joinK k1 k2 of JoinK k f1 f2 -> C k (addK k (f1 a1) (f2 a2))
+  C k1 a1 * C k2 a2 = case joinK k1 k2 of JoinK k f1 f2 -> C k (mulK k (f1 a1) (f2 a2))
+  C k1 a1 - C k2 a2 = case joinK k1 k2 of JoinK k f1 f2 -> C k (subK k (f1 a1) (f2 a2))
+  negate (C k x) = C k (negateK k x)
+  abs (C k x) = C k (absK k x)
+  signum (C k x) = C Q (signumK k x)
+  fromInteger = C Q . fromInteger
+
+instance Fractional Construct where
+  C k1 a1 / C k2 a2 = case joinK k1 k2 of JoinK k f1 f2 -> C k (divK k (f1 a1) (f2 a2))
+  recip (C k a) = C k (recipK k a)
+  fromRational = C Q
+
+-- |The type of exceptions thrown by impossible 'Construct' operations.
+data ConstructException =
+  -- |'toRational' was given an irrational constructible number.
+  ConstructIrrational |
+  -- |'sqrt' was given a negative constructible number.
+  ConstructSqrtNegative |
+  -- |'**' was given an exponent that is not a dyadic rational, or a transcendental function was called.
+  Unconstructible String
+  deriving (Eq, Ord, Typeable)
+
+instance Show ConstructException where
+  showsPrec _ ConstructIrrational = showString "cannot convert irrational Construct to rational"
+  showsPrec _ ConstructSqrtNegative = showString "Construct sqrt: negative argument"
+  showsPrec _ (Unconstructible s) = showString s . showString " is not constructible"
+
+instance Exception ConstructException
+
+{- |
+This partial 'Floating' instance only supports 'sqrt' and '**' where
+the exponent is a dyadic rational.  Passing a negative number to
+'sqrt' will throw the 'ConstructSqrtNegative' exception.  All other
+operations will throw the 'Unconstructible' exception.
+-}
+instance Floating Construct where
+  sqrt (C k a)
+    | sgnK k a == LT = throw ConstructSqrtNegative
+    | otherwise = case sqrtK k a of
+        Nothing -> C (Sqrt k a) (SqrtElt (zeroK k) (fromRationalK k 1))
+        Just b -> C k b
+  pi = throw (Unconstructible "pi")
+  exp = throw (Unconstructible "exp")
+  log = throw (Unconstructible "log")
+  a ** b = go (numerator b') (denominator b') where
+    b' = toRational b
+    go p q = let (n, p') = divMod p q in
+      (if n >= 0 then a^n else 1/a^(-n))*go' p' q
+    go' 0 _ = 1
+    go' p q = case divMod q 2 of
+      (q', 0) -> sqrt (go p q')
+      _ -> throw (Unconstructible "(** non-dyadic-rational)")
+  logBase = throw (Unconstructible "logBase")
+  sin = throw (Unconstructible "sin")
+  tan = throw (Unconstructible "tan")
+  cos = throw (Unconstructible "cos")
+  asin = throw (Unconstructible "asin")
+  atan = throw (Unconstructible "atan")
+  acos = throw (Unconstructible "acos")
+  sinh = throw (Unconstructible "sinh")
+  tanh = throw (Unconstructible "tanh")
+  cosh = throw (Unconstructible "cosh")
+  asinh = throw (Unconstructible "asinh")
+  atanh = throw (Unconstructible "atanh")
+  acosh = throw (Unconstructible "acosh")
+
+{- |
+This 'Real' instance only supports 'toRational' on numbers that are in
+fact rational.  'toRational' on an irrational number will throw the
+'ConstructIrrational' exception.
+-}
+instance Real Construct where
+  toRational = either id (\_ -> throw ConstructIrrational) . deconstruct
+
+instance RealFrac Construct where
+  properFraction (C Q x) = (m, C Q y) where (m, y) = properFraction x
+  properFraction x = (fromInteger m, x - fromInteger m)
+    where m = search ((> x) . fromInteger) - 1
+
+instance Enum Construct where
+  succ = (+ 1)
+  pred = (subtract 1)
+  toEnum = fromIntegral
+  fromEnum = fromInteger . truncate
+  enumFrom n = n `seq` (n : enumFrom (n + 1))
+  enumFromThen n m = n `seq` m `seq` (n : enumFromThen m (m + m - n))
+  enumFromTo n m = takeWhile (<= m) (enumFrom n)
+  enumFromThenTo e1 e2 e3 = takeWhile predicate (enumFromThen e1 e2) where
+    predicate | e2 >= e1 = (<= e3)
+              | otherwise = (>= e3)
+
+mk :: a -> a -> Complex a
+mk = (:+)
+
+toPair :: Complex a -> (a, a)
+toPair (x :+ y) = (x, y)
+
+deriveComplexF ''Complex ''Construct 'mk 'toPair
+
+{- |
+Evaluate a floating-point approximation for a constructible number.
+
+To improve numerical stability, addition of numbers with different
+signs is avoided using quadratic conjugation.
+-}
+fromConstruct :: Floating a => Construct -> a
+fromConstruct (C k a) = fromConstructK k a
diff --git a/LICENSE b/LICENSE
new file mode 100644
--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,29 @@
+Copyright © 2013, Anders Kaseorg <andersk@mit.edu>
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are
+met:
+
+• Redistributions of source code must retain the above copyright
+  notice, this list of conditions and the following disclaimer.
+
+• Redistributions in binary form must reproduce the above copyright
+  notice, this list of conditions and the following disclaimer in the
+  documentation and/or other materials provided with the distribution.
+
+• Neither the name of Anders Kaseorg nor the names of other
+  contributors may be used to endorse or promote products derived from
+  this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+“AS IS” AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
diff --git a/Setup.hs b/Setup.hs
new file mode 100644
--- /dev/null
+++ b/Setup.hs
@@ -0,0 +1,2 @@
+import Distribution.Simple
+main = defaultMain
diff --git a/constructible.cabal b/constructible.cabal
new file mode 100644
--- /dev/null
+++ b/constructible.cabal
@@ -0,0 +1,35 @@
+name:                constructible
+version:             0.1
+synopsis:            Exact computation with constructible real numbers
+description:
+  The constructible reals are the subset of the real numbers that can
+  be represented exactly using field operations (addition,
+  subtraction, multiplication, division) and positive square roots.
+  They support exact computations, equality comparisons, and ordering.
+homepage:            http://andersk.mit.edu/haskell/constructible/
+license:             BSD3
+license-file:        LICENSE
+author:              Anders Kaseorg <andersk@mit.edu>
+maintainer:          Anders Kaseorg <andersk@mit.edu>
+copyright:           © 2013 Anders Kaseorg
+category:            Math
+build-type:          Simple
+cabal-version:       >=1.8
+
+library
+  exposed-modules:     Data.Real.Constructible
+  build-depends:
+    arithmoi >=0.1,
+    base ==4.*,
+    binary-search >=0.0,
+    complex-generic >=0.1
+  ghc-options:       -Wall
+
+source-repository head
+  type:                git
+  location:            https://github.com/andersk/haskell-constructible
+
+source-repository this
+  type:                git
+  location:            https://github.com/andersk/haskell-constructible
+  tag:                 0.1