"do" notation and ">>"

Koen Claessen [email protected]
Sun, 14 Apr 2002 13:20:04 +0200 (MET DST)


 |   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.

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

/Koen.

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