Re: [Haskell-cafe] Referential Transparency and Monads

[Brandon S. Allbery KF8NH]

On 2009 Apr 9, at 22:47, Brandon S. Allbery KF8NH wrote:

> -- these implicitly are considered to return a modified RealWorld
> readFile :: RealWorld -> (String,RealWorld)
> writeFile :: RealWorld -> ((),RealWorld)
> threadDelay :: RealWorld -> ((),RealWorld)
>
> main :: RealWorld -> ((),RealWorld)
> main state =
>   case readFile state "/my_file" of
>     (s,state') ->
>        case writeFile state' "/my_file" "Hello, world!\n" of
>          (_,state'') ->
>             case threadDelay state'' 10000 of
>               (_,state'') -> writeFile "/my_file" s   

Sorry, the last line should be:

>               (_,state''') -> writeFile state''' "/my_file" s



> type IO a = State RealWorld a



--
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allb...@kf8nh.com
system administrator [openafs,heimdal,too many hats] allb...@ece.cmu.edu
electrical and computer engineering, carnegie mellon university    KF8NH

This code is nothing more nor less than the translation of Jonathan Cast's code, using a presumptive into "linear" de-monadified code. Clearly the state is threaded into main, through all the calls, and the final state returned; the state threading is what maintains the referentially transparent model.

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