Friday, November 2, 2012

syntactic sucralose

In programming languages there is a concept "syntactic sugar". As wikipedia describes it In computer science, syntactic sugar is syntax within a programming language that is designed to make things easier to read or to express.
In some languages (especially the MongoDB shell), there is the reverse concept. There are language features that are present to "make it work" but have no bearing on anything in the program's context. I call these syntactic sucralose. They are the only things available to get the desired result, but leave a bitter taste in your mouth afterwards.
The case that riled me up today was the $unset "operator" in Mongodb's shell interface. To unset (eliminate a name value pair in a MongoDB document), you write something of the following form for the second argument of the .update method.
{$unset :{foo : 1}}. The 1 in this case is a mandatory positional parameter that has no relation to the current value of foo. In fact you could put anything that is a legitimate value (string, date, objectID, integer, boolean...) in place of the 1.  In fact whatever is placed there is evaluated.  So for example, the code fragment

,{$unset :{foo,x++}} does result in both foo becoming unset and x being incremented.

Even if the value is an unbound variable name, it still is acceptable.

Lots of scope for mischief. This should come with a government health warning

1 comment:

  1. Huh?
    > var x=0;
    Mon Aug 12 00:03:11.244 JavaScript execution failed: SyntaxError: Unexpected token ,
    Invalid modifier specified: $unset

    What in the world are you talking about?