On 29/12/2011 18:04, Donn Cave wrote:
Quoth Steve Horne<sh006d3...@blueyonder.co.uk>,
...
Anyway, if you're using IO actions, your code is not referentially
transparent and is therefore impure - by your own definition of
"impure". Causing side-effects may not be pedantically the issue, but
the mix of causing and reacting to them - ie interacting with the
"outside" - clearly means that some of your function results are
dependent on what's happening "outside" your program. That includes
side-effects "outside" your program yet caused by program program.
No, code can be referential transparent and pure and at the same time
use IO actions. In order to understand that, you need to untangle the
notion you describe above as "function result" from Haskell function
value. We can talk endlessly about what your external/execution results
might be for some IO action, but at the formulaic level of a Haskell
program it's a simple function value, e.g., IO Int.
To me, that only makes sense if you never run the compiled program - if
the executable file is just an interesting artifact that you generated
using a Haskell "interpreter".
In reality, the behaviour of IO actions is part of Haskell. The precise
meaning of primitive Haskell IO actions is defined. The effects of
compositing to build larger IO actions is defined. The Haskell language
and compiler take responsibility for meaning of IO actions. The effect
of executing those actions, including the returned values, is absolutely
relevant to the behaviour of the program.
You can make the argument that "the world" is a parameter. Well - in C,
"the world" can be considered an implicit parameter. In any case, this
only gives referential transparency by what I'd call deceptive
definition. Only a tiny piece of "the world" is relevant to your
program. You've buried the relevant in a mass of the irrelevant, very
much like a less-than-transparent politician. Your interaction is no
more or less likely to have bugs depending on whether you define this as
transparent or not - arguing about the definition is besides the point.
If a program that causes and is sensitive to side-effects - that
interacts with the outside world - is referentially transparent, then
referential transparency has no relevant meaning.
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