On 2008 Dec 27, at 15:16, Cristiano Paris wrote:
On Sat, Dec 27, 2008 at 4:50 PM, Hans van Thiel
<hthiel.c...@zonnet.nl> wrote:
However, some functions in Haskell may have side effects, like
printing
something on the screen, updating a database, or producing a random
number. These functions are called 'actions' in Haskell.
Functions may return "actions", which are a special kind of value even
though they are no different to Haskell from values of any other type.
There is another way to think about it (which happens to also be the
way GHC implements it): an "action" is just a partially applied
function. In the case of IO, the missing value is the state, which
has the opaque internal type RealWorld# that can't normally be
instantiated.
(unsafePerformIO works by using an internal function which synthesizes
a value of type RealWorld#. Since all the IO code *and* the code
generator in GHC assume this can't be done, unsafePerformIO is capable
of confusing things rather badly. In particular, you need to tell GHC
to inline any code using unsafePerformIO because otherwise it can make
incorrect assumptions and very strange behavior will result.
UnsafeInterleaveIO is even sneakier: it clones the hidden RealWorld#
value, which brings its own kinds of confusion into the mix.)
In the normal course of events, your program becomes a long chain of
unapplied functions (think foo. bar . baz . (...)), waiting for
something to inject a RealWorld# into it. This is the value of
"main". Conceptually, the runtime then injects a RealWorld# and the
program runs. (In reality, GHC knows that code doesn't actually have
to be generated to carry RealWorld# values around, so the generated
program ends up looking more or less normal.)
--
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allb...@kf8nh.com
system administrator [openafs,heimdal,too many hats] allb...@ece.cmu.edu
electrical and computer engineering, carnegie mellon university KF8NH
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