Whose...It

The whose clause allows you to filter a result or set of results based on specified relevance criteria. It has the form:

<expression> whose <condition>

For example:

The special keyword it refers to the elements of the list – in this case the collection of numbers – and is bound only within the filter expression. The filter expression is executed once for every value in the filtered property, with it referring to each result in turn. The results where the filter clause evaluates to True are included in the output list. Note that it always refers to the list immediately to the left of the whose statement.

The it keyword can also refer to objects that are not part of a whose clause:

Here, it takes on the values in the list, one at a time.

You can also use parentheses to define the scope of the whose-it objects. A judicious use of parentheses can ensure proper results while improving readability. For example, the following examples show how subtle rearrangement of whose clauses can change the output significantly:

These expressions both go character-by-character through the word 'banana' and return the text preceding each character. Because it returns the text before the character, it returns the blank before 'b' and stops at the final 'a' with 'banan'. The expressions both return the same values, but the second one makes it more clear what it refers to, namely 'banana'. Since 'banana' always has an 'n', this expression returns all the specified substrings.

These two expressions are equivalent, but the second one shows more explicitly what it refers to, namely the characters of the word banana. The 'n' appears twice in banana, and so two substrings are returned.

These two expressions do the same thing, but the second one makes it obvious that it refers to the text preceding the character. Thus only the initial substrings of 'banana' that contain an 'n' are returned.

In practical usage, you could use whose clauses to filter folders:

Or you can put the whose clause at the end of the expression, which makes the object of it more explicit and might be easier to read:

If the filtered property is singular, the result of the whose clause is singular. If the filtered property is a plural type, the result is a plural type.

This singular property evaluates to true if there is an active display device on the client computer.

This plural expression returns a list of system files whose names start with 'x'.

As it loops through the plural values, the expression in the filter might attempt to evaluate a non-existent object. By itself, such an expression would cause an error such as:

Singular expression refers to nonexistent object.

But in the case of a whose clause, the non-existent value is simply ignored and gets excluded from the resulting set. As a side effect, this feature allows you to examine an object for existence before you attempt to inspect it (and cause an error). As an example, here's a Relevance clause that triggers an existence error:

But, by placing this clause inside a whose statement, you can avoid the error:

Watch the video about the Whose-It in Relevance Language to have more information about this clause.