Null object: Difference between revisions
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There is a builtin NULL, however it is equivalent to the integer 0 and will trigger a type check if assigned to a variable declared as string or sequence. In most programs the zero-length string/sequence (""/{}) suffices,
but if you want a variable that can be a string/sequence or NULL, but not other arbitrary integer/float values, use something like the following user-defined types:
--s = 1 -- error▼
<!--<lang Phix>-->
<span style="color: #008080;">type</span> <span style="color: #000000;">nullableString</span><span style="color: #0000FF;">(</span><span style="color: #004080;">object</span> <span style="color: #000000;">o</span><span style="color: #0000FF;">)</span>
<span style="color: #008080;">return</span> <span style="color: #004080;">string</span><span style="color: #0000FF;">(</span><span style="color: #000000;">o</span><span style="color: #0000FF;">)</span> <span style="color: #008080;">or</span> <span style="color: #000000;">o</span><span style="color: #0000FF;">=</span><span style="color: #004600;">NULL</span>
<span style="color: #008080;">end</span> <span style="color: #008080;">type</span>
<span style="color: #000000;">nullableString</span> <span style="color: #000000;">s</span>
<span style="color: #000000;">s</span> <span style="color: #0000FF;">=</span> <span style="color: #008000;">"hello"</span>
<span style="color: #000000;">s</span> <span style="color: #0000FF;">=</span> <span style="color: #004600;">NULL</span>
<span style="color: #000080;font-style:italic;">--s = 1 -- error
▲ --s = {1,2,3} -- error</span>
<span style="color: #008080;">type</span> <span style="color: #000000;">nullableSequence</span><span style="color: #0000FF;">(</span><span style="color: #004080;">object</span> <span style="color: #000000;">o</span><span style="color: #0000FF;">)</span>
<span style="color: #008080;">return</span> <span style="color: #004080;">sequence</span><span style="color: #0000FF;">(</span><span style="color: #000000;">o</span><span style="color: #0000FF;">)</span> <span style="color: #008080;">or</span> <span style="color: #000000;">o</span><span style="color: #0000FF;">=</span><span style="color: #004600;">NULL</span>
<span style="color: #008080;">end</span> <span style="color: #008080;">type</span>
<span style="color: #000000;">nullableSequence</span> <span style="color: #000000;">q</span>
<span style="color: #000000;">q</span> <span style="color: #0000FF;">=</span> <span style="color: #0000FF;">{</span><span style="color: #000000;">1</span><span style="color: #0000FF;">,</span><span style="color: #000000;">2</span><span style="color: #0000FF;">,</span><span style="color: #000000;">3</span><span style="color: #0000FF;">}</span>
<span style="color: #000000;">q</span> <span style="color: #0000FF;">=</span> <span style="color: #008000;">"string"</span> <span style="color: #000080;font-style:italic;">-- fine (strings are a subset of sequences)</span>
<span style="color: #000000;">q</span> <span style="color: #0000FF;">=</span> <span style="color: #004600;">NULL</span>
<span style="color: #000080;font-style:italic;">--q = 1 -- error</span>
<!--</lang>-->
See also [[Undefined_values#Phix|Undefined_values]]
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