Fix signum implementation (#280)

* Fix signum implementation

* Add changelog entry

* Reimplement w/o 'otherwise'

* Apply suggestions from code review

Co-authored-by: Harry Garrood <harry@garrood.me>

* Update src/Data/Ord.purs

Co-authored-by: Harry Garrood <harry@garrood.me>

* Adjust doc comment to use sized zeros rendering

* Add test for signum

* Add missing period.

Co-authored-by: Harry Garrood <harry@garrood.me>

* Ensure signum test functions correctly

* Do test via `show (1.0/zero)` with clarifying test

Co-authored-by: Harry Garrood <harry@garrood.me>

Author: JordanMartinez

CHANGELOG.md

@@ -8,6 +8,7 @@ Breaking changes:
 - Migrated FFI to ES Modules (#287 by @kl0tl and @JordanMartinez)
 - Change Generic Rep's `NoConstructors` to newtype `Void` (#282 by @JordanMartinez)
 - Replaced polymorphic proxies with monomorphic `Proxy` (#281, #288 by @JordanMartinez)
+- Fix `signum zero` to return `zero` (#280 by @JordanMartinez)
 
 New features:
 

src/Data/Ord.purs

@@ -216,10 +216,16 @@ 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.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 +152,10 @@ testRecordInstances = do
   assert "Record top" $
     (top :: { a :: Boolean }).a
     == top
+
+testSignum :: AlmostEff
+testSignum = do
+  assert "Clarifies what 'signum positive zero' test is doing" $ show (1.0/0.0) == "Infinity"
+  assert "signum positive zero" $ show (1.0/(signum 0.0)) == "Infinity"
+  assert "Clarifies what 'signum negative zero' test is doing" $ show (1.0/(-0.0)) == "-Infinity"
+  assert "signum negative zero" $ show (1.0/(signum (-0.0))) == "-Infinity"