enumFromThenTo for Doubles

[Simon Peyton Jones]

Sounds somewhat plausible.  By all means give it a try.

S

-----Original Message-----
From: ghc-devs [mailto:ghc-devs-bounces at haskell.org] On Behalf Of Andrew Farmer
Sent: 10 August 2016 04:22
To: ghc-devs at haskell.org
Subject: enumFromThenTo for Doubles

Noticed this today:

ghci> let xs = [0.0,0.1 .. 86400.0] in maximum xs
86400.0000005062

enumFromThenTo is implemented by numericEnumFromThenTo:

https://github.com/ghc/ghc/blob/a90085bd45239fffd65c01c24752a9bbcef346f1/libraries/base/GHC/Real.hs#L227

Which probably accumulates error in numericEnumFromThen with the (m+m-n):

numericEnumFromThen n m = n `seq` m `seq` (n : numericEnumFromThen m (m+m-n))

Why not define numericEnumFromThen as:

numericEnumFromThen n m = let d = m - n in d `seq` go d n
where go delta x = x `seq` (x : go delta (x + delta))

(or with BangPatterns)

numericEnumFromThen n m = go (m - n) n
where go !delta !x = x : go delta (x + delta)

Seems like we'd save a lot of subtractions by using the worker function.
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