Re: [Haskell-cafe] Let, Equations, and Monad

Il giorno 27 ott 2015, alle ore 13:32, PATRICK BROWNE ha scritto:

{- 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

Hi, I may not be able to correctly answer your question, but I’d make you notice that you don’t need all that nesting to express the same thing. Since those computations are pure, you don’t need to sequence them in that way: eligible account = all (< 0) [account1, account2, account3, account4] where account1 = deposit 100 account account2 = withdraw 200 account1 account3 = deposit 100 account2 account4 = withdraw 300 account3 account5 = deposit 1000 account4 Greetings, Nicola