"do" notation and ">>"

[Koen Claessen]

I wrote:

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

Simon Peyton-Jones answered:

 | It might, conceivably.  But the H98 report doesn't
 | seem the right place to try to tweak full laziness.
 | So I'm going to leave the report as it is.  Hugs and
 | GHC have changed to match.

I do not understand what full laziness has to do with all
this! The big question is, in the following:

  f = do <expr1>
         <expr2>

Should <expr2> be shared among different calls to f?  It is
clear that <expr1> will, but <expr2> will not be shared,
using the current translation used by GHC and Hugs.

Maybe I should be a bit more concrete; Here is a little
example program:

>>>
main =
  do print "start"
     writeFile "apa" (show [1..])
<<<

When translating the do-notation using >>, we blow out of
heap space (in both Hugs and GHC (*)). The code then looks
as follows:

>>>
main1 =
  print "start" >>
  writeFile "apa" (show [1..])
<<<

When translating the do-notation using >>=, we do not blow
out of heap. The code then looks as follows:

>>>
main2 =
  print "start" >>= \_ ->
  writeFile "apa" (show [1..])
<<<

The reason for this difference is that the computation
"writeFile "apa" (show [1..])" is kept in memory in "main1"
and not in "main2".

So, concretely, the fix in Hugs and GHC will possibly break
a number of programs. Specifically programs that for example
produce a lot of output which does not depend on any
run-time information.

Regards,
/Koen.

(*) Since GHC garbage collects CAFs, we have to add an extra
reference to "main", for example:

  main = do print "start"
            writeFile "apa" (show [1..])
            main

--
Koen Claessen
http://www.cs.chalmers.se/~koen
Chalmers University, Gothenburg, Sweden.