[Haskell-cafe] On hGetContents semi-closenesscloseness

[Ertugrul Soeylemez]

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Brandon S Allbery KF8NH <allbery.b at gmail.com> wrote:

> Haskell is actually what manufacturing folks call "just in time";
> things are evaluated when they are needed.  Usually this means that
> when you output something, anything needed to compute that output will
> be done then.  The exceptions are things like
> Control.Exception.evaluate (which you can treat as doing output but
> without *actually* outputting anything), mentioned above, plus you can
> indicate that some computation must be evaluated before another by
> means of Prelude.seq.  You can also declare a type as being strict by
> prefixing an exclamation mark (so the runtime will always evaluate a
> computation before binding it), and with the BangPatterns extension
> you can also declare a pattern match binding as strict the same way.

Note that pattern matches are strict by default.  In fact, a pattern
match is the preferred way to force evaluation.  Bang patterns only make
sure that variables (i.e. wildcards, which wouldn't be evaluated
otherwise) in a pattern match are evaluated to WHNF:

  case expr of
    Just x  -> ...
    Nothing ->

This is strict in the Maybe constructors, but non-strict in the argument
of Just.  When using a bang pattern,

  Just (!x) -> ...

the match is also strict in the argument of Just.  A bang pattern is
really just a shortcut for using 'seq':

  Just x -> seq x $ ...

or as some people prefer to write it:

  Just x | seq x True -> ...


Greets,
Ertugrul


- -- 
nightmare = unsafePerformIO (getWrongWife >>= sex)
http://ertes.de/
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