Niladic function

In APL syntax, a niladic function is a special type of function that takes no arguments and is evaluated immediately when it is encountered in a statement. It cannot be used as an operand because this evaluation happens before operator binding, or any other evaluation step such as stranding. For this reason operators are always defined to take non-niladic operands. Niladic is considered one of three possible function valences along with monadic and dyadic.

System variables like <syntaxhighlight lang=apl inline>⎕TS</source> (time stamp) might be considered instances of niladic functions but are usually not described this way.

In most dialects, a niladic function cannot be created as a derived function or dfn. However, an APL programmer can define a tradfn which takes no arguments: <syntaxhighlight lang=apl> ∇ res←set_a a ← 'a' res ← 1 ∇ </source> After defining such a function, it can be evaluated simply by writing its name, with no need to indicate any arguments. A niladic function is the only possible named entity with this behaviour: for any other kind of value, writing the name alone in a statement will simply return that value, but for a niladic function it evaluates the function and returns the result. <syntaxhighlight lang=apl>

     a

VALUE ERROR: Undefined name: a

     a
     ∧
     set_a

1

     a

a </source>