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Today's Topics:
1. let , equations, and monads (PATRICK BROWNE)
----------------------------------------------------------------------
Message: 1
Date: Wed, 28 Oct 2015 14:59:09 +0000
From: PATRICK BROWNE <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: [Haskell-beginners] let , equations, and monads
Message-ID:
<cagflrkf6t1cbultwhhktgunceq+-9d5txp5p8osz4m8hedg...@mail.gmail.com>
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{- From Learn Haskell Fast and Hard : 4.3.1. Maybe is a monad
http://yannesposito.com/Scratch/en/blog/Haskell-the-Hard-Way/#maybe-monad
Concerning the code below I have the following questions:
1) Are eligibleLet and eligibleEquational operationally equivalent (i.e.
perform the same operations) and/or semantically equivalent(i.e.
Church-Rosser)?
2) Apart from the return type of Maybe instead of Bool how does
eligibleMonad differ from eligibleLet?
Regards,
Pat
-}
deposit value account = account + value
withdraw value account = account - value
-- You are eligible for a bonus only if your sequence of transactions stays
out of the red.
eligibleLet :: (Num a,Ord a) => a -> Bool
eligibleLet account =
let account1 = deposit 100 account in
if (account1 < 0)
then False
else
let account2 = withdraw 200 account1 in
if (account2 < 0)
then False
else
let account3 = deposit 100 account2 in
if (account3 < 0)
then False
else
let account4 = withdraw 300 account3 in
if (account4 < 0)
then False
else
let account5 = deposit 1000 account4 in
if (account5 < 0)
then False
else
True
-- No let expressions, hence intermediate calculations are performed
multiple times.
-- Should terminates on first point of failure
eligibleEquational :: (Num a,Ord a) => a -> Bool
eligibleEquational account = if (deposit 100 account) < 0 then False else
if (withdraw 200 (deposit 100 account)) < 0
then False else
if (deposit 100 (withdraw 200 (deposit 100
account))) < 0 then False else
if (withdraw 300 (deposit 100 (withdraw 200
(deposit 100 account)))) < 0 then False else
if (deposit 1000 (withdraw 300 (deposit 100
(withdraw 200 (deposit 100 account))))) < 0 then False else
True
-- Monadic version
depositM :: (Num a) => a -> a -> Maybe a
depositM value account = Just (account + value)
withdrawM :: (Num a,Ord a) => a -> a -> Maybe a
withdrawM value account = if (account < value)
then Nothing
else Just (account - value)
eligibleMonad :: (Num a, Ord a) => a -> Maybe Bool
eligibleMonad account = do
account1 <- depositM 100 account
account2 <- withdrawM 200 account1
account3 <- depositM 100 account2
account4 <- withdrawM 300 account3
account5 <- depositM 1000 account4
Just True
main = do
print $ eligibleLet 300 -- True
print $ eligibleLet 299 -- Falsen
print $ eligibleEquational 300 -- True
print $ eligibleEquational 299 -- False
print $ eligibleMonad 300 -- Just True
print $ eligibleMonad 299 -- Nothing
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