[Haskell-cafe] Top Level <-

[Sittampalam, Ganesh]

> > Contrived example follows:
> 
> > module Module1 (mod1) where
> > import Module2
> >
> > glob1 :: IORef Int
> > glob1 <- mod2 >>= newIORef
> 
> > mod1 :: IO Int
> > mod1 = readIORef glob1
> 
> > module Module2 (mod2) where

> > import Module1

> > glob2 :: IORef Int
> > glob2 <- mod1 >>= newIORef
> 
> > mod2 :: IO Int
> > mod2 = readIORef glob2

> This is illegal because you're only allowed to use ACIO
> in top level <- bindings and readIORef isn't (and clearly
> could not be) ACIO.

(made a couple of changes to quoted example; added import
statements and explicit export lists)

Even though I never call writeIORef on glob1 or glob2, and
can change the example as above so we don't export them, so
it's impossible to ever do so?

As an alternative, consider

module Module1 (mod1) where
import Module2

glob1 :: Int
glob1 <- return $! mod2

mod1 :: Int
mod1 = glob1
 
module Module2 (mod2) where

import Module1

glob2 :: Int
glob2 <- return $! mod1
 
mod2 :: Int
mod2 = glob2

Even more artificial, of course.

Arguably both of these cases are not ACIO simply because
of the non-termination effects, but it's not obvious to
me how you tell just by looking at either one's code together
with the declared API of the other. Is anything strict
automatically forbidden by ACIO?

Cheers,

Ganesh




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