From aa48ef307a7127f83189af37170f73492b4805ef Mon Sep 17 00:00:00 2001
From: sof <unknown>
Date: Sun, 5 Oct 1997 21:33:42 +0000
Subject: [PATCH] [project @ 1997-10-05 21:33:42 by sof] Do not use default
methods for Enum.enumFromThenTo for Double and Float instances
---
ghc/lib/ghc/PrelNum.lhs | 33 ++++++++++++++++++++++-----------
1 file changed, 22 insertions(+), 11 deletions(-)
diff --git a/ghc/lib/ghc/PrelNum.lhs b/ghc/lib/ghc/PrelNum.lhs
index 6cf3dcd55f21..95724b63e3c4 100644
--- a/ghc/lib/ghc/PrelNum.lhs
+++ b/ghc/lib/ghc/PrelNum.lhs
@@ -666,30 +666,41 @@ instance Show Double where
%* *
%*********************************************************
-The Enum instances for Floats and Doubles are slightly unusual.
-The `toEnum' function truncates numbers to Int. The definitions
-of enumFrom and enumFromThen allow floats to be used in arithmetic
+The @Enum@ instances for Floats and Doubles are slightly unusual.
+The @toEnum@ function truncates numbers to Int. The definitions
+of @enumFrom@ and @enumFromThen@ allow floats to be used in arithmetic
series: [0,0.1 .. 1.0]. However, roundoff errors make these somewhat
dubious. This example may have either 10 or 11 elements, depending on
how 0.1 is represented.
+NOTE: The instances for Float and Double do not make use of the default
+methods for @enumFromTo@ and @enumFromThenTo@, as these rely on there being
+a `non-lossy' conversion to and from Ints. Instead we make use of the
+1.2 default methods (back in the days when Enum had Ord as a superclass)
+for these (@numericEnumFromTo@ and @numericEnumFromThenTo@ below.)
+
\begin{code}
instance Enum Float where
- toEnum = fromIntegral
- fromEnum = fromInteger . truncate -- may overflow
- enumFrom = numericEnumFrom
- enumFromThen = numericEnumFromThen
+ toEnum = fromIntegral
+ fromEnum = fromInteger . truncate -- may overflow
+ enumFrom = numericEnumFrom
+ enumFromThen = numericEnumFromThen
+ enumFromThenTo = numericEnumFromThenTo
instance Enum Double where
- toEnum = fromIntegral
- fromEnum = fromInteger . truncate -- may overflow
- enumFrom = numericEnumFrom
- enumFromThen = numericEnumFromThen
+ toEnum = fromIntegral
+ fromEnum = fromInteger . truncate -- may overflow
+ enumFrom = numericEnumFrom
+ enumFromThen = numericEnumFromThen
+ enumFromThenTo = numericEnumFromThenTo
numericEnumFrom :: (Real a) => a -> [a]
numericEnumFromThen :: (Real a) => a -> a -> [a]
+numericEnumFromThenTo :: (Real a) => a -> a -> a -> [a]
numericEnumFrom = iterate (+1)
numericEnumFromThen n m = iterate (+(m-n)) n
+numericEnumFromThenTo n m p = takeWhile (if m >= n then (<= p) else (>= p))
+ (numericEnumFromThen n m)
\end{code}
--
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