[Haskell-cafe] The Transient monad

[Alberto G. Corona]

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 vl...@dikini.net

 jeanchristophe.min...@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.


 http://www.haskell.org/ghc/docs/6.10.4/html/libraries/base/System-Mem-StableName.html
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Re: [Haskell-cafe] The Transient monad

[Twan van Laarhoven]

Alberto G. Corona wrote:

Hi haskell cafe:

concerning Stable Names 

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 doesn't really give 'the same result every time', though. For 
example:


* let sn x = hashStableName $ makeStableName x
* let x = replicate 3 'x' in (,) $ sn x * evaluate x * sn x
(18,17)

After x is evaluated in this example, its stable name changes.

Perhaps instead of Transient we could use a name that makes it more clear what 
is going on, perhaps ObservingEvaluation. That could also include exception 
handling, by the way.




makeStableName :: a - Transient (StableName a)


Why not wrap it up in a class?

class Monad m = MonadObservingEvaluation m where
-- what should go here? perhaps:
liftOE :: ObservingEvaluation a - m a

Then we can have

makeStableName :: MonadObservingEvaluation m = a - m (StableName a)

Which works both in IO and the new monad.


Twan
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