From: Fearless Fool <r@...>
Date: 2010-09-09T06:25:59+09:00
Subject: Re: Problem with Hash of Arrays

@Gary, @Chris:

You're both right, of course: a *real* programmer will *always* use 
has_key?() (or equivalent) to check for a non-empty slot in a hash 
table!  Or will she?

Consider the following example:

1:  sentence = ParserModule.parse(string)
2:  if (p = sentence[:preposition])
3:     ...

Depending on how the 'sentence' hash table is constructed, line 2 may or 
may not modify the state of the hash table, and YOU CANNOT KNOW unless 
you look inside the ParserModule.  Why is it bad if the state of the 
hash table changes unexpectedly?  I dunno: maybe your code depends on 
the number of elements in the hash, or (as the code above suggests) the 
distinction between a nil slot and a non-nil slot is important.

But for line 2 to modify the state of the hash table is a clear 
violation of "The Principle of (Matz's) Least Surprise".

- ff

P.S.: If you claim that it's better to write this as:

1:  sentence = ParserModule.parse(string)
2. if sentence.has_key?(:preposition)
3.   p = sentence[:preposition]
      ...

... I'll sic the DRY police on you.  :)
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