Q: Forcing repeated evaluation

[Martin Norbäck]

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tor 2002-09-12 klockan 11.27 skrev Jan Kybic:
> Hello,
>         I have another question regarding the optimisation of Haskell cod=
e:
> I have a relatively inexpensive function generating a long list, imagine
> something like (I simplified a lot):
>=20
> l =3D [ i*i*i | i <- [0..n] ]   -- for very large n
>=20
> This long list is consumed several times in the program:
>=20
> x1 =3D f1 l
> x2 =3D f2 x1 l
> x3 =3D f3 x2 l
>=20
> I found that the list l is calculated just once and that the
> computational time is dominated by the allocations and garbage
> collection. I want to try to force l to be generated on-the-fly
> every time it is needed, to see if it improves performance.
> What is a good way to do it? Would something like
>=20
> unsafePerformIO $ return l
>=20
> do the job? Isn'it there any flag for the compiler (ghc) to suggest
> this optimisation? Thank you for your feedback.

The easiest way is to make it a function

l _ =3D [ i*i*i | i <- [0..n] ]   -- for very large n

x1 =3D f1 (l ())
x2 =3D f2 x1 (l ())
x3 =3D f3 x2 (l ())

() can be any value really.

Regards,

	Martin

--=20
Martin Norb=E4ck          d95mback@dtek.chalmers.se             =20
Kapplandsgatan 40       +46 (0)708 26 33 60                   =20
S-414 78  G=D6TEBORG      http://www.dtek.chalmers.se/~d95mback/
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