Design criteria for languages 
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 Design criteria for languages

In a posting yesterday on "APL Implementors Considered Harmful",
I neglected to include one desirable criteria we can use to
measure the cost of a proposed language extension:

   How much does the extension cost to those who do NOT use it?

As an example, consider the behavior of singleton extension in
scalar functions.
The expression  Scalar+Vector always produces a Vector. This is Good.
The expression  Singleton+Vector usually produces a Vector. Unless
the Vector has exactly 1 element, in which case the expression produces
a Singleton.

This extension to singletons, therefore, has two Evil Effects on
people who NEVER use the extension (And on those who DO use it, as well):

a. The interpreter has to make special checks on EACH execution to see
   if this is the singleton case, for ALL scalar functions.

b. A person maintaining a system has to be aware that this expression
   can produce a non-vector result, based on the LENGTH, not the RANK,
   of one argument. This can lead to unforseen bugs, difficulty in
   analysis, and so on.

Is singleton extension worth it? I think not.

We can use similar techniques to measure the impact of other proposed
and extant language features.
Bob



Mon, 27 Dec 1999 03:00:00 GMT  
 
 [ 1 post ] 

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