logBase is:exact

logBase :: Floating a => a -> a -> a
base Prelude Numeric GHC.Float, hedgehog Hedgehog.Internal.Prelude, ghc GHC.Prelude.Basic, base-compat Prelude.Compat, protolude Protolude Protolude.Base, relude Relude.Numeric, rio RIO.Prelude, base-prelude BasePrelude, classy-prelude ClassyPrelude, Cabal-syntax Distribution.Compat.Prelude, basic-prelude CorePrelude, universum Universum.Base, clash-prelude Clash.HaskellPrelude, ghc-lib-parser GHC.Prelude.Basic, prelude-compat Prelude2010, ghc-internal GHC.Internal.Float GHC.Internal.Numeric, rebase Rebase.Prelude, xmonad-contrib XMonad.Config.Prime, constrained-categories Control.Category.Constrained.Prelude Control.Category.Hask, copilot-language Copilot.Language.Prelude, LambdaHack Game.LambdaHack.Core.Prelude, cabal-install-solver Distribution.Solver.Compat.Prelude, faktory Faktory.Prelude, vcr Imports, verset Verset, yesod-paginator Yesod.Paginator.Prelude, distribution-opensuse OpenSuse.Prelude, hledger-web Hledger.Web.Import, termonad Termonad.Prelude
logBase :: C a => a -> a -> a
numeric-prelude Algebra.Transcendental NumericPrelude NumericPrelude NumericPrelude.Numeric NumericPrelude.Numeric
logBase :: ExpField a => a -> a -> a
numhask NumHask NumHask.Algebra.Field
log to the base of 
>>> logBase 2 8
2.9999999999999996
logBase :: Floating a => Dimensionless a -> Dimensionless a -> Dimensionless a
dimensional Numeric.Units.Dimensional Numeric.Units.Dimensional.Prelude
Takes the logarithm of the second argument in the base of the first. 
>>> logBase _2 _8
3.0
logBase :: ExpField a => BackpropFunc (a, a) a
inf-backprop InfBackprop Prelude.InfBackprop
Natural logarithm differentiable function.

Examples of usage

>>> import Prelude (Float)

>>> import InfBackprop (call, derivative)

>>> call logBase (2, 8) :: Float
3.0

>>> import Debug.SimpleExpr.Expr (variable)

>>> x = variable "x"

>>> n = variable "n"

>>> derivative logBase (n, x)
((1·(-(log(x))·(1/(log(n)·log(n)))))·(1/n),(1·(1/log(n)))·(1/x))