[Haskell-cafe] Thunks and GHC pessimisation

[oleg at okmij.org]

Tom Ellis wrote:
> To avoid retaining a large lazy data structure in memory it is useful to
> hide it behind a function call.  Below, "many" is used twice.  It is hidden
> behind a function call so it can be garbage collected between uses. 

As you discovered, it is quite challenging to ``go against the grain''
and force recomputation. GHC is quite good at avoiding
recomputation. This is a trade-off, of time vs space. For large
search tree, it is space that is a premium, and laziness and similar
strategies are exactly the wrong trade-off. 

The solution (which I've seen in some of the internal library code) is
to confuse GHC with extra functions:
        http://okmij.org/ftp/Haskell/misc.html#memo-off

So, eventually it is possible to force recomputation. But the solution
leaves a poor taste -- fighting a compiler is never a good idea. So,
this is a bug of sort -- not the bug of GHC, but of lazy
evaluation. Lazy evaluation is not the best evaluation strategy. It is
a trade-off, which suits a large class of problems and punishes
another large class of problems.