"do" notation and ">>"

[David Feuer]

On Sun, Apr 14, 2002, Koen Claessen wrote:
> 
>  |   do {e ; stmts}   =   e >> do {stmts}
> 
> With the risk of being late at this, and with the risk of
> repeating something that has already been said (because I
> have been away), I will give my two euro-cents.
> 
> I remember a discussion at the Haskell mailing list about
> the possibility of creating a nasty space leak with this
> translation. After all, there is a difference between:
> 
>   <e1> >> <e2>
> 
> which, for the default definition of >> expands to an
> expression which is operationally equivalent to:
> 
>   <e1> >>= let x = <e2> in \_ -> x,
> 
> and:
> 
>   <e1> >>= \_ -> <e2>.
> 
> In the first case, x is shared between successive
> invocations, but in the second case, e2 is recomputed.
> 
> I remember that the example involved a definition like:
> 
>   f = do ...; print [1..]
> 
> In the translation in the report, the list [1..] is shared
> between all invocations of f, whereas the current GHC
> translation does not.


If I understand this correctly, this only comes up when full laziness is
used.  Since full laziness turns space-efficiency upside-down, it does
not seem to make much sense to try to tweak the Report to make things
work better with full laziness.

Note:  I think it could be pretty cool to have something like full
laziness that depended on programmer annotations:

f p = let x = e1 in e2
would not save the value of x across calls to f, but
f p = letfl x = e1 in e2
would save it.

Something like this could be used for those cases where full laziness
makes programs easier to read/write/understand without bothering people
in cases where it is a bad idea.


> 
> So, changing the translation in GHC might actually introduce
> a very nasty space leak in existing programs!

-- 
Night.  An owl flies o'er rooftops.  The moon sheds its soft light upon
the trees.
David Feuer