Fix signum implementation

CHANGELOG.md

@@ -5,6 +5,7 @@ Notable changes to this project are documented in this file. The format is based
 ## [Unreleased]
 
 Breaking changes:
+- Fix `signum zero` to return `zero` (#280 by @JordanMartinez)
 
 New features:
 

src/Data/Ord.purs

@@ -214,10 +214,16 @@ between low hi x
 abs :: forall a. Ord a => Ring a => a -> a
 abs x = if x >= zero then x else negate x
 
--- | The sign function; always evaluates to either `one` or `negate one`. For
--- | any `x`, we should have `signum x * abs x == x`.
+-- | The sign function; returns `one` if the argument is positive,
+-- | `negate one` if the argument is negative, or `zero` if the argument is `zero`.
+-- | For floating point numbers with signed zeroes, when called with a zero,
+-- | this function returns the argument in order to preserve the sign.
+-- | For any `x`, we should have `signum x * abs x == x`.
 signum :: forall a. Ord a => Ring a => a -> a
-signum x = if x >= zero then one else negate one
+signum x =
+  if x < zero then negate one
+  else if x > zero then one
+  else x
 
 -- | The `Ord1` type class represents totally ordered type constructors.
 class Eq1 f <= Ord1 f where

test/Test/Main.js

@@ -42,3 +42,9 @@ exports.testNumberShow = function(showNumber) {
       ]);
   };
 };
+
+exports.objectIs = function(l) {
+  return function(r) {
+    return Object.is(l, r);
+  };
+};

test/Test/Main.purs

@@ -2,7 +2,7 @@ module Test.Main where
 
 import Prelude
 import Data.HeytingAlgebra (ff, tt, implies)
-import Data.Ord (abs)
+import Data.Ord (abs, signum)
 import Test.Data.Generic.Rep (testGenericRep)
 import Test.Utils (AlmostEff, assert)
 
@@ -15,6 +15,7 @@ main = do
     testIntDegree
     testRecordInstances
     testGenericRep
+    testSignum
 
 foreign import testNumberShow :: (Number -> String) -> AlmostEff
 
@@ -151,3 +152,13 @@ testRecordInstances = do
   assert "Record top" $
     (top :: { a :: Boolean }).a
     == top
+
+testSignum :: AlmostEff
+testSignum = do
+  assert "signum positive zero" $ objectIs (signum 0.0) 0.0
+  assert "signum negative zero" $ objectIs (signum (-0.0)) (-0.0)
+
+-- Seems to be only way to check whether zero is `-0.0` or `0.0`
+-- See "Case 2" in
+-- https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is#using_object.is
+foreign import objectIs :: Number -> Number -> Boolean