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.

System variables like ⎕TS might be considered instances of niladic functions but are usually not described this way.

A niladic function cannot be created as a derived function or dfn. However, an APL programmer can define a tradfn which takes no arguments:

∇ res←set_a
a ← 'a'
res ← 1
∇

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.

      a
VALUE ERROR: Undefined name: a
      a
      ∧
      set_a
1
      a
a

APL syntax [edit]
General	Comparison with traditional mathematics ∙ Precedence ∙ Tacit programming (Train, Hook, Split composition)
Array	Numeric literal ∙ String ∙ Strand notation ∙ Object literal ∙ Array notation (design considerations)
Function	Argument ∙ Function valence ∙ Derived function ∙ Derived operator ∙ Niladic function ∙ Monadic function ∙ Dyadic function ∙ Ambivalent function ∙ Defined function (traditional) ∙ Dfn ∙ Function train
Operator	Operand ∙ Operator valence ∙ Tradop ∙ Dop ∙ Derived operator
Assignment	Multiple ∙ Indexed ∙ Selective ∙ Modified
Other	Function axis ∙ Bracket indexing ∙ Branch ∙ Statement separator ∙ Quad name ∙ System command ∙ User command ∙ Keyword ∙ Dot notation ∙ Function-operator overloading ∙ Control structure ∙ Comment

Niladic function

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