# Difference between revisions of "Output/Input"

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+ | === <u>Clearing away the smoke and mirrors</u> === |
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<div style="border-left:1px solid lightgray; padding: 1em" alt="blockquote"> |
<div style="border-left:1px solid lightgray; padding: 1em" alt="blockquote"> |
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− | Still, today, over 25 years after the introduction of the concept of monads to the world of functional programming, beginning functional programmers struggle to grasp the concept of monads. This struggle is exemplified by the numerous blog posts about the effort of trying to learn about monads. From our own experience we notice that even at university level, bachelor level students often struggle to comprehend monads and consistently score poorly on monad-related exam questions. |
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+ | The implementation in GHC uses the following one: |
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− | Considering that the concept of monads is not likely to disappear from the functional programming landscape any time soon, it is vital that we, as the functional programming community, somehow overcome the problems novices encounter when first studying monads. |
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+ | <haskell> |
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+ | type IO a = World -> (a, World) |
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+ | </haskell> |
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− | <tt>[https://pms.cs.ru.nl/iris-diglib/src/getContent.php?id=2017-Steenvoorden-SupportLearning Visual Support for Learning Monads], Tim Steenvoorden, Jurriën Stutterheim, Erik Barendsen and Rinus Plasmeijer.</tt> |
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+ | An <code>IO</code>computation is a function that (logically) takes the state of the world, and returns a modified world as well as the return value. Of course, GHC does not actually pass the world around; instead, it passes a dummy “token,” to ensure proper sequencing of actions in the presence of lazy evaluation, and performs input and output as actual side effects! |
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+ | |||

+ | <tt>[https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.168.4008&rep=rep1&type=pdf A History of Haskell: Being Lazy With Class], Paul Hudak, John Hughes, Simon Peyton Jones and Philip Wadler.</tt> |
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</div> |
</div> |
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− | ...so where are students of Haskell most likely to have their first encounter with this irksome interface? Here's a hint: |
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+ | ...so what starts out as an I/O action of type: |
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− | <div style="border-left:1px solid lightgray; padding: 1em" alt="blockquote"> |
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+ | <haskell> |
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− | <code>IO</code> is the monad you cannot avoid. |
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+ | World -> (a, World) |
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+ | </haskell> |
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− | <tt>[https://image.slidesharecdn.com/functionalconf2019-whyishaskellsohard2-191116135003/95/why-is-haskell-so-hard-and-how-to-deal-with-it-53-638.jpg Why Haskell is so HARD? (And how to deal with it)]; Saurabh Nanda.</tt> |
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+ | is changed by GHC to approximately: |
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+ | |||

+ | <haskell> |
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+ | () -> (a, ()) |
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+ | </haskell> |
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+ | |||

+ | As the returned unit-value <code>()</code> contains no useful information, that type can be simplified further: |
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+ | |||

+ | <haskell> |
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+ | () -> a |
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+ | </haskell> |
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+ | |||

+ | Why "approximately"? Because "logically" a function in Haskell has no observable effects. |
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+ | |||

+ | ---- |
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+ | === <u>Previously seen</u> === |
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+ | |||

+ | Variations of the type <code>() -> a</code> have appeared elsewhere: |
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+ | |||

+ | * page 2 of 13 in [https://fi.ort.edu.uy/innovaportal/file/20124/1/22-landin_correspondence-between-algol-60-and-churchs-lambda-notation.pdf A Correspondence Between ALGOL 60 and Church's Lambda-Notation: Part I] by Peter Landin: |
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+ | :{| |
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+ | |<div style="border-left:1px solid lightgray; padding: 1em" alt="blockquote"> |
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+ | |||

+ | The use of <code>λ</code>, and in particular (to avoid an irrelevant bound variable) of <code>λ()</code> , to delay and possibly avoid evaluation is exploited repeatedly in our model of ALGOL 60. A function that requires an argument-list of length zero is called a ''none-adic'' function. |
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</div> |
</div> |
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+ | <sup> </sup> |
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+ | <haskell> |
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+ | (\ () -> …) :: () -> a |
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+ | </haskell> |
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+ | |} |
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+ | |||

+ | * page 3 of [https://www.cs.bham.ac.uk/~udr/papers/assign.pdf Assignments for Applicative Languages] by Vipin Swarup, Uday S. Reddy and Evan Ireland: |
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+ | :{| |
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+ | |<div style="border-left:1px solid lightgray; padding: 1em" alt="blockquote"> |
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+ | A value of type <code>Obs 𝜏</code> is called an ''observer''. Such a value observes (i.e. views or inspects) a state and returns a value of type <code>𝜏</code>. [...] An observer type <code>Obs 𝜏</code> may be viewed as an implicit function space from the set of states to the type <code>𝜏</code>. |
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+ | </div> |
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+ | <sup> </sup> |
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+ | <haskell> |
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+ | type Obs tau = State -> tau |
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+ | </haskell> |
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+ | |} |
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+ | |||

+ | * [https://image.slidesharecdn.com/lazyio-120422092926-phpapp01/95/lazy-io-15-728.jpg page 15] of ''Non-Imperative Functional Programming] by Nobuo Yamashita: |
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+ | |||

+ | :{| |
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+ | <haskell> |
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+ | type a :-> b = OI a -> b |
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+ | </haskell> |
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+ | |} |
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+ | |||

+ | * [http://h2.jaguarpaw.co.uk/posts/mtl-style-for-free MTL style for free] by Tom Ellis: |
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+ | |||

+ | :{| |
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+ | <haskell> |
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+ | data Time_ a = GetCurrentTime (UTCTime -> a) |
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+ | |||

+ | data Lock_ a = AcquireLock (Maybe Lock -> a) NominalDiffTime Key |
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+ | | RenewLock (Maybe Lock -> a) NominalDiffTime Lock |
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+ | | ReleaseLock (() -> a) Lock |
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+ | </haskell> |
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+ | |} |
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+ | |||

+ | * page 2 of [https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.128.9269&rep=rep1&type=pdf Unique Identifiers in Pure Functional Languages] by Péter Diviánszky. |
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+ | :{| |
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+ | |<div style="border-left:1px solid lightgray; padding: 1em" alt="blockquote"> |
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+ | [...] The type <code>Id</code> can be hidden by the synonym data type |
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+ | <pre> |
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+ | :: Create a :== Id -> a |
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+ | </pre> |
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+ | </div> |
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+ | <sup> </sup> |
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+ | <haskell> |
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+ | type Create a = Id -> a |
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+ | </haskell> |
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+ | |} |
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+ | |||

+ | * page 26 of [https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.91.3579&rep=rep1&type=pdf How to Declare an Imperative] by Philip Wadler: |
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+ | :{| |
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+ | |<div style="border-left:1px solid lightgray; padding: 1em" alt="blockquote"> |
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+ | The type <code>'a io</code> is represented by a function expecting a dummy argument of type unit and returning a value of type <code>'a</code>. |
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+ | <pre> |
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+ | type 'a io = unit -> a |
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+ | </pre> |
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+ | </div> |
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+ | <sup> </sup> |
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+ | <haskell> |
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+ | type Io a = () -> a |
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+ | </haskell> |
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+ | |} |
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+ | |||

+ | * [https://stackoverflow.com/questions/6647852/haskell-actual-io-monad-implementation-in-different-language/6706442#6706442 ysdx's answer] to [https://stackoverflow.com/questions/6647852/haskell-actual-io-monad-implementation-in-different-language this SO question]: |
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+ | :{| |
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+ | |<div style="border-left:1px solid lightgray; padding: 1em" alt="blockquote"> |
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+ | Let's say you want to implement <code>IO</code> in SML : |
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+ | <pre> |
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+ | structure Io : MONAD = |
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+ | struct |
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+ | type 'a t = unit -> 'a |
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+ | ⋮ |
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+ | end |
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+ | </pre> |
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+ | </div> |
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+ | <sup> </sup> |
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+ | <haskell> |
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+ | type T a = () -> a |
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+ | </haskell> |
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+ | |} |
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+ | |||

+ | * [https://stackoverflow.com/questions/45136398/is-the-monadic-io-construct-in-haskell-just-a-convention/45141523#45141523 luqui's answer] to [https://stackoverflow.com/questions/45136398/is-the-monadic-io-construct-in-haskell-just-a-convention this SO question]: |
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+ | :{| |
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+ | |<haskell> |
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+ | newtype IO a = IO { runIO :: () -> a } |
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+ | </haskell> |
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+ | |} |
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+ | |||

+ | * [https://stackoverflow.com/questions/15418075/the-reader-monad/15419592#15419592 luqui's answer] to [https://stackoverflow.com/questions/15418075/the-reader-monad this SO question]: |
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+ | :{| |
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+ | |<haskell> |
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+ | newtype Supply r a = Supply { runSupply :: r -> a } |
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+ | </haskell> |
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+ | |} |
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+ | |||

+ | Of these, it is the implementation of <code>OI a</code> in Yamashita's [https://hackage.haskell.org/package/oi oi] package which is most interesting as its values are ''monousal'' - once used, their contents remain constant. This single-use property also appears in the implementation of the abstract <code>decision</code> type described by F. Warren Burton in [https://academic.oup.com/comjnl/article-pdf/31/3/243/1157325/310243.pdf Nondeterminism with Referential Transparency in Functional Programming Languages]. |
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+ | |||

+ | ---- |
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+ | === <code>IO</code><u>, redefined</u> === |
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+ | |||

+ | Based on these and other observations, a reasonable generalisation of these examples would be <code>OI -> a</code>, which then implies: |
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+ | |||

+ | <haskell> |
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+ | type IO a = OI -> a |
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+ | </haskell> |
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+ | |||

+ | Using Burton's ''pseudodata'' approach: |
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+ | |||

+ | <haskell> |
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+ | -- abstract; single-use I/O-access mediator |
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+ | data Exterior |
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+ | getchar :: Exterior -> Char |
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+ | putchar :: Char -> Exterior -> () |
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+ | |||

+ | -- from section 2 of Burton's paper |
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+ | data Tree a = Node { contents :: a, |
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+ | left :: Tree a, |
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+ | right :: Tree a } |
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+ | |||

+ | -- utility definitions |
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+ | type OI = Tree Exterior |
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+ | |||

+ | getChar' :: OI -> Char |
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+ | getChar' = getchar . contents |
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+ | |||

+ | putChar' :: Char -> OI -> () |
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+ | putChar' c = putchar c . contents |
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+ | |||

+ | part :: OI -> (OI, OI) |
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+ | parts :: OI -> [OI] |
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+ | |||

+ | part t = (left t, right t) |
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+ | parts t = let !(t1, t2) = part t in |
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+ | t1 : parts t2 |
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+ | </haskell> |
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+ | |||

+ | Of course, in an actual implementation <code>OI</code> would be abstract like <code>World</code>, and for similar reasons. This allows for a simpler implementation for <code>OI</code> and its values, instead of being based on (theoretically) infinite structured values like binary trees. That simplicity has benefits for the <code>OI</code> interface, in this case: |
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+ | |||

+ | <haskell> |
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+ | data OI |
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+ | part :: OI -> (OI, OI) |
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+ | getChar' :: OI -> Char |
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+ | putChar' :: Char -> OI -> () |
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+ | </haskell> |
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+ | <sup> </sup> |
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− | If bachelor-level students are often struggling to comprehend the monadic interface, then it has ''no place whatsoever'' in any introductory Haskell course! I/O in Haskell must therefore be totally independent of the monadic interface. The introduction of this interface, including its basic operations, should be deferred to an advanced course of study where it can join functors, applicatives, arrows and maybe even category theory itself: only then should students be introduced to monadic I/O. |
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+ | ---- |
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− | A [https://www.interaction-design.org/literature/article/kiss-keep-it-simple-stupid-a-design-principle simple] general-purpose model of I/O for non-strict functional languages remains an [[Open research problems|open research problem]]... |
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+ | See also: |
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− | [[Category:Rants and comments]] |
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+ | * [[IO, partible-style]] |
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+ | * [[IO then abstraction]] |
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+ | * [[Open research problems]] |

## Revision as of 13:52, 2 November 2021

__Clearing away the smoke and mirrors__

The implementation in GHC uses the following one:

```
type IO a = World -> (a, World)
```

An `IO`

computation is a function that (logically) takes the state of the world, and returns a modified world as well as the return value. Of course, GHC does not actually pass the world around; instead, it passes a dummy “token,” to ensure proper sequencing of actions in the presence of lazy evaluation, and performs input and output as actual side effects!

`A History of Haskell: Being Lazy With Class, Paul Hudak, John Hughes, Simon Peyton Jones and Philip Wadler.`

...so what starts out as an I/O action of type:

```
World -> (a, World)
```

is changed by GHC to approximately:

```
() -> (a, ())
```

As the returned unit-value `()`

contains no useful information, that type can be simplified further:

```
() -> a
```

Why "approximately"? Because "logically" a function in Haskell has no observable effects.

__Previously seen__

Variations of the type `() -> a`

have appeared elsewhere:

- page 2 of 13 in A Correspondence Between ALGOL 60 and Church's Lambda-Notation: Part I by Peter Landin:

The use of

`λ`

, and in particular (to avoid an irrelevant bound variable) of`λ()`

, to delay and possibly avoid evaluation is exploited repeatedly in our model of ALGOL 60. A function that requires an argument-list of length zero is called a*none-adic*function.^{ }(\ () -> …) :: () -> a

- page 3 of Assignments for Applicative Languages by Vipin Swarup, Uday S. Reddy and Evan Ireland:

A value of type

`Obs 𝜏`

is called an*observer*. Such a value observes (i.e. views or inspects) a state and returns a value of type`𝜏`

. [...] An observer type`Obs 𝜏`

may be viewed as an implicit function space from the set of states to the type`𝜏`

.^{ }type Obs tau = State -> tau

- page 15 of
*Non-Imperative Functional Programming] by Nobuo Yamashita:*

type a :-> b = OI a -> b

- MTL style for free by Tom Ellis:

data Time_ a = GetCurrentTime (UTCTime -> a) data Lock_ a = AcquireLock (Maybe Lock -> a) NominalDiffTime Key | RenewLock (Maybe Lock -> a) NominalDiffTime Lock | ReleaseLock (() -> a) Lock

- page 2 of Unique Identifiers in Pure Functional Languages by Péter Diviánszky.

[...] The type

`Id`

can be hidden by the synonym data type:: Create a :== Id -> a

^{ }type Create a = Id -> a

- page 26 of How to Declare an Imperative by Philip Wadler:

The type

`'a io`

is represented by a function expecting a dummy argument of type unit and returning a value of type`'a`

.type 'a io = unit -> a

^{ }type Io a = () -> a

Let's say you want to implement

`IO`

in SML :structure Io : MONAD = struct type 'a t = unit -> 'a ⋮ end

^{ }type T a = () -> a

newtype IO a = IO { runIO :: () -> a }

newtype Supply r a = Supply { runSupply :: r -> a }

Of these, it is the implementation of `OI a`

in Yamashita's oi package which is most interesting as its values are *monousal* - once used, their contents remain constant. This single-use property also appears in the implementation of the abstract `decision`

type described by F. Warren Burton in Nondeterminism with Referential Transparency in Functional Programming Languages.

`IO`

__, redefined__

Based on these and other observations, a reasonable generalisation of these examples would be `OI -> a`

, which then implies:

```
type IO a = OI -> a
```

Using Burton's *pseudodata* approach:

```
-- abstract; single-use I/O-access mediator
data Exterior
getchar :: Exterior -> Char
putchar :: Char -> Exterior -> ()
-- from section 2 of Burton's paper
data Tree a = Node { contents :: a,
left :: Tree a,
right :: Tree a }
-- utility definitions
type OI = Tree Exterior
getChar' :: OI -> Char
getChar' = getchar . contents
putChar' :: Char -> OI -> ()
putChar' c = putchar c . contents
part :: OI -> (OI, OI)
parts :: OI -> [OI]
part t = (left t, right t)
parts t = let !(t1, t2) = part t in
t1 : parts t2
```

Of course, in an actual implementation `OI`

would be abstract like `World`

, and for similar reasons. This allows for a simpler implementation for `OI`

and its values, instead of being based on (theoretically) infinite structured values like binary trees. That simplicity has benefits for the `OI`

interface, in this case:

```
data OI
part :: OI -> (OI, OI)
getChar' :: OI -> Char
putChar' :: Char -> OI -> ()
```

^{ }

See also: