[Haskell-beginners] Re: [Haskell-cafe] Just how unsafe is unsafe

Peter Verswyvelen bugfact at gmail.com
Thu Feb 5 17:16:56 EST 2009

I do have asked myself the question whether a "really random generating"
function could be regarded as "pure" somehow (actually would a true random
function still be a mathematical function?)
E.g. the function would return a true (not pseudo) random number,
practically unpredictable (e.g. hardware assisted, using some physical
phenomenon, e.g. using atmospheric noise or something). So you surely won't
get referential transparency but since the function is really random, this
would be correct behavior?

Of course you could just put this random generator in the IO monad, but
certain algorithms- like Monte Carlo - intuitively don't seem to operate in
a IO monad to me.

Okay, just some thoughts from someone who knows absolutely nothing about
category theory or advanced computer science, so don't shoot me ;-)

On Thu, Feb 5, 2009 at 10:36 PM, Jake McArthur <jake at pikewerks.com> wrote:

> Hash: SHA1
> Andrew Wagner wrote:
> | I understand that unsafeXXX allows impurity, which defiles our ability
> | to reason logically about haskell programs like we would like to. My
> | question is, to what extent is this true?
> My opinion is that unsafeXXX is acceptable only when its use is
> preserved behind an abstraction that is referentially transparent and
> type safe. Others may be able to help refine this statement.
> | Suppose we had a module, UnsafeRandoms, which had a function that would
> | allow you to generate a different random number every time you call it.
> | The semantics are relatively well-defined, impurity is safely sectioned
> | off in its own impure module, which is clearly labeled as such. How much
> | damage does this do?
> This does not preserve referential transparency, so by my criteria above
> this is an unacceptable use of an unsafe function. One reason it's a bad
> idea is that it removes determinism, which may be very important for
> testability.
> - - Jake
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