From 45afba05c6d0fca1e1b2d101c097a7738bb73b25 Mon Sep 17 00:00:00 2001
From: simonpj <unknown>
Date: Tue, 18 Jun 2002 09:31:06 +0000
Subject: [PATCH] [project @ 2002-06-18 09:31:05 by simonpj]
 -------------------------- 	Deal with NaN and Infinity 
 --------------------------

This commit arranges that

* GHC.Real exports	infinity   :: Rational = 1 :% 0
			notANumber :: Rational = 0 :% 0

* GHC.Float.fromRat converts these Rational non-numbers into
  their Float/Double equivalents


As a result, arith015/arith016 start to work again.  We can
read NaN and Infinity into Float/Double.
---
 GHC/Float.lhs    |   19 ++++++++++++-------
 GHC/Real.lhs     |    7 +++++++
 Text/Read/Lex.hs |   17 +----------------
 3 files changed, 20 insertions(+), 23 deletions(-)

diff --git a/GHC/Float.lhs b/GHC/Float.lhs
index 382ef64..5c68de4 100644
--- a/GHC/Float.lhs
+++ b/GHC/Float.lhs
@@ -675,14 +675,18 @@ fromRat x = x'
 Now, here's Lennart's code (which works)
 
 \begin{code}
-{-# SPECIALISE fromRat :: 
-	Rational -> Double,
-	Rational -> Float #-}
+{-# SPECIALISE fromRat :: Rational -> Double,
+			  Rational -> Float #-}
 fromRat :: (RealFloat a) => Rational -> a
-fromRat x 
-  | x == 0    =  encodeFloat 0 0 		-- Handle exceptional cases
-  | x <  0    =  - fromRat' (-x)		-- first.
-  | otherwise =  fromRat' x
+
+-- Deal with special cases first, delegating the real work to fromRat'
+fromRat (n :% 0) | n > 0  =  1/0	-- +Infinity
+		 | n == 0 =  0/0	-- NaN
+		 | n < 0  = -1/0	-- -Infinity
+
+fromRat (n :% d) | n > 0  = fromRat' (n :% d)
+		 | n == 0 = encodeFloat 0 0 		-- Zero
+		 | n < 0  = - fromRat' ((-n) :% d)
 
 -- Conversion process:
 -- Scale the rational number by the RealFloat base until
@@ -693,6 +697,7 @@ fromRat x
 -- a first guess of the exponent.
 
 fromRat' :: (RealFloat a) => Rational -> a
+-- Invariant: argument is strictly positive
 fromRat' x = r
   where b = floatRadix r
         p = floatDigits r
diff --git a/GHC/Real.lhs b/GHC/Real.lhs
index 13be142..be30073 100644
--- a/GHC/Real.lhs
+++ b/GHC/Real.lhs
@@ -41,6 +41,13 @@ default ()		-- Double isn't available yet,
 \begin{code}
 data  (Integral a)	=> Ratio a = !a :% !a  deriving (Eq)
 type  Rational		=  Ratio Integer
+
+infinity, notANumber :: Rational
+infinity   = 1 :% 0
+notANumber = 0 :% 0
+
+-- Use :%, not % for Inf/NaN; the latter would 
+-- immediately lead to a runtime error, because it normalises. 
 \end{code}
 
 
diff --git a/Text/Read/Lex.hs b/Text/Read/Lex.hs
index f2c0469..01ca6ac 100644
--- a/Text/Read/Lex.hs
+++ b/Text/Read/Lex.hs
@@ -36,7 +36,7 @@ import GHC.Show( Show(.. ), showChar, showString,
 		 isSpace, isAlpha, isAlphaNum,
 		 isOctDigit, isHexDigit, toUpper )
 import GHC.Real( Ratio(..), Integral, Rational, (%), fromIntegral, fromRational, 
-		 toInteger, (^), (^^) )
+		 toInteger, (^), (^^), infinity, notANumber )
 import GHC.Float( Float, Double )
 import GHC.List
 import GHC.Show( ShowS, shows )
@@ -284,21 +284,6 @@ lexString =
 -- ---------------------------------------------------------------------------
 --  Lexing numbers
 
-infinity, notANumber :: Rational
-infinity   = 1 :% 0
-notANumber = 0 :% 0
-
--- Use :%, not % for Inf/NaN, the latter would 
--- immediately lead to a runtime error. 
---
--- Note that
---     isInfinite (read "Infinity" :: Float)
---  or
---     isInfinite (fromRational (1 :% 0))
--- still don't work, because fromRational is not OK for those cases.
--- 
--- The whole Infinity/NaN story is a bit murky to me
-
 type Base   = Int
 type Digits = [Int]
 
-- 
1.7.10.4