[Haskell-cafe] Re: The Transient monad

[Ben Franksen]

+1

Alberto G. Corona  wrote:
> Hi haskell cafe:
> 
> concerning Stable Names
> 
>
http://www.haskell.org/ghc/docs/6.10.4/html/libraries/base/System-Mem-StableName.html
> 
> 
> makeStableName :: a -> IO (StableName a)
> 
> I Did not test fully my proposal, and I´m thinking aloud, Just to inpire
> others and fish some ideas;
> 
> The IO in makeStableName  suggest more side effects than makeStableName
> really do. But still the call isn't pure.
> 
> For calls such are makeStableName that gives a different result the FIRST
> time they are called but return the same result every time in the same
> session,  I suggest to use a Transient monad:
> 
> 
> makeStableName :: a -> Transient (StableName a)
> 
> The key here is to maintain the programmer aware that it is not pure, but
> there are no IO and that  the results have no meaning from session to
> session.
> 
> Instance Monad Transient where
>      Transient x ↠ f =  f  x
>      return x = Transient x
> 
> We can Transient`ize IO calls by means of an implicit memoization:
> 
> liftT:: IO a -> Transient a
> liftT= < whatever memoization code>
> liftT2=....
> liftT3=....
> 
> Memorization then is embedded in the monad transformation
> This may be more elegant than  IO plus unsafePerformIO and is more
> informative for the programmer. The transition form IO to pure can be done
> in two steps, see below
> 
>   Instead of unsafePerformIO,  we can use:
> 
> unsafePurifyTransient :: Transient a -> a
> unsafePurifyTransient  (Transient x) = x
> 
> for the inherently transient calls
> 
> A safer version of unsafePerformIO using implicit memoization could be:
> unsafePerformIOT :: IO a -> a
> unsafePerformIOT = unsafePurifyTransient  .  liftT
> 
> unsafePerformIOT guatantee that it returns the same value in the same
> session.
> 
> 
> 
> 
> 2009/12/8 Vladimir Zlatanov <vlado at dikini.net>
> 
>> <jeanchristophe.mincke at gmail.com> wrote:
>> > I think lisp like symbols could be quite useful in the context of
>> embedded
>> > DSL to create ... well... symbols that can be interpreted as variables
>> > in that DSL.
>>
>> Well for such use-case you could use either stable names, or recode
>> them into a state monad- you will get either a global i.e. lisp like
>> unique symbols, or their equivalent within a state context. Or some
>> variation of the code posted previously in this thread.