Running out of memory in a simple monad

[Simon Marlow]

Alastair Reid writes:

> The workaround is simple enough: add a dummy argument to the CAF (so
> that it is not a CAF any more):
>=20
>    main _ =3D loop 50000
>=20
> and then specify the extra argument when invoking it:
>=20
>    main ()
>=20
> (This is a pretty standard optimisation technique: we're trading time
> to recompute a result for the space taken to store the result.  Coming
> from other languages where actions (i.e., monadic computations) are
> not first class values, this is a bit surprising but, from a Haskell
> perspective, it is completely uniform.)

Careful: this isn't guaranteed to turn a CAF into a function.  In
particular, GHC will "optimise away" this trick when optimisation is
turned on (and perhaps even when it isn't).  The point is that adding
dummy arguments isn't really a technique that should be relied upon.

You might well argue that there ought to be a way to control the
operational behaviour of the program w.r.t. CAFs (and in fact sharing in
general), and I'd be inclined to agree.

Also, GHCi retains CAFs in the same way as Hugs, the difference is that
GHCi can be configured to throw away the results after evaluation (:set
+r).

Cheers,
	Simon