Re: Ada functions versus arrays (i.e. () vs [] )

[mph@lion.inmos.co.uk (Mike Harrison)]

In article <6281@bd.sei.cmu.edu> firth@sei.cmu.edu (Robert Firth) writes:

>As a second example, consider a dictionary used by a spelling checker.
>There is a function Check(W) that checks whether W is in the dictionary.
>Many implementations of this function are adaptive: they use a temporary
>cache of some kind that is dynamically reorganised so that words
>occurring frequently in the document in question float to the front.
>Over a long document, this can substantially speed the checking process.
>Again, this is a benign and proper use of side effects within a function.

I question whether this is really a side-effect.
Clearly it is a side-effect at the level of the implementation, in that it
causes a change of state from a call of the function, but if you view the 
function as a mapping from elements of its domain to elements of its range
then there is no observable change at the appropriate level of abstraction.

This is not a disagreement with Robert's point, because the Ada code of the 
function body will clearly cause side-effects.
It is rather an attempt to point out that 'pernicious' side-effects are those
which are visible at the same level of abstraction as the function definition,
rather than its implementation.

The problem is writing a compiler which can recognise the distinction.

Mike,




Michael P. Harrison - Software Group - Inmos Ltd. UK.
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