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Make IsMissing work more intuitively

KevW

Pop quiz!

Question 1

Suppose you have a function GetCurrentValue which gets the current value of user certain interface objects, and returns missing (.) for others. Suppose you have a list of interface objects (AllInterfaceObjects).

Will this work?

UserValues = Transform Each({object}, AllInterfaceObjects, GetCurrentValue(object));
FilteredValues = Filter Each({value}, UserValues, !Is Missing(value));

Answer:

Depends on what kind of values GetCurrentValue returns.
Suppose GetCurrentValue returns (checkbox << Get Selected Indices()) for checkbox items. If you apply IsMissing to the list of indices you do not get a boolean literal, you get a list of booleans.

!IsMissing({1,2,3}) = {1,1,1}

Because a list of booleans is not True, Filter Each will remove it! That means FilteredValue will not contain your list, even though your list was not missing.

Depends on what kind of values GetCurrentValue returns. Suppose GetCurrentValue returns (checkbox << Get Selected Indices()) for checkbox items. If you apply IsMissing to the list of indices you do not get a boolean literal, you get a list of booleans.!IsMissing({1,2,3}) = {1,1,1}Because a list of booleans is not True, Filter Each will remove it! That means FilteredValue will not contain your list, even though your list was not missing.

Question 2

As you may or may not know, Create Database Connection is a function for creating database connections. It returns a database connection on success, and missing on failure.

Does this work?

connection = Create Database Connection(magicString);
if( !Is Missing(connection), 
    RunSomeQuery(connection),
    Print("I have failed to connect")
);

Answer:

Nope. Is Missing returns an error, complaining that it expects numeric values.

My Proposal

Make the use of missing (.) and Is Missing more disciplined.

Do not magically thread the function through lists, tell me if the variable holding the list is missing or not.
If Is Missing cannot handle your success, do not use missing to signal failure.

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