[0,0.1..1.0] != (map (/ 10) [0..10]) ?

IRC の #Haskell に貼った小ネタ

QuickCheck 用に aribitrary を実装する際などに，本当に [0,0.1..1.0] となる数のリストが欲しいのであれば，そのまま 小数のリテラルを使うのではなく，整数のリストを map (/ 10) で割ると期待通りの値が取れるよ，というお話

---

if you wants number list of [0,0.1..1.0] , not use it but map (/ 10) [0..10] .

numberliteral-is-liar.hs

-- Is List Literal with Number liar ?
-- 
-- ;-)
--
-- This Sample Code Depend on test-framework, and QuickCheck 2
-- try this
-- cabal install test-framework
-- cabal install test-framework-th
-- cabal install test-framework-quickcheck2
-- cabal install QuickCheck
-- runhaskell numberliteral-is-liar.hs

{-# LANGUAGE TemplateHaskell #-}

module Main ( main ) where

import Test.Framework.TH
import Test.Framework
import Test.Framework.Providers.QuickCheck2
import Test.QuickCheck

main = $(defaultMainGenerator)


contains xs x = any (== x) xs

fixedList = map (/ 10) [1..10]

-- Do you expect to pass this test property ?
prop_floating_number_list =
  forAll ( elements [0,0.1..1.0] ) $ contains fixedList

-- ;-)
prop_true_arbitrary_value =
  forAll ( fmap (/ 10) $ elements [1..10] ) $ contains fixedList

-- Why? try ghci ;-)
-- > map (/ 10) [1..10]
-- > [0,0.1..1.0]