Dfn
A dfn (contraction of direct function or dynamic function, pronounced "dee fun") is an alternative way to define a function and operator, invented by John Scholes. A dfn operator can also be called a dop (pronounced "dee op").
John Scholes was responsible for numerous presentations and publications on and about dfns, and until his passing he maintained the dfns workspace, a collection of dfns examples.
As of 2020, dfns are fully implemented in Dyalog APL, NARS2000, ngn/apl, dzaima/APL, and partially in GNU APL, although not all dialects implement lexical scoping, in contrast to the dynamic scoping of tradfns. In other words, a dfn cannot "see" locals of its caller, but can see locals of its definer.
Wikipedia includes a thorough treatment of dfns.
Contents
Examples
{⍵*0.5} 16 ⍝ square root
4
3 {⍵*÷⍺} 27 ⍝ ⍺th root
3
⍝ Multiline dfn with optional left parameter and conditional result
root←{
⍺←2 ⍝ square root by default
⍺=0:0 ⍝ return zero if zeroth root
⍵*÷⍺ ⍝ result
}
Error-guards
Dyalog APL dfns support error-guards for processing errors by error codes.[1]
In the following example, there are two error-guards for the error code 11 (DOMAIN ERROR):[2]
Gravity←{
G←6.6743E¯11 ⍝ gravitational constant
11::'N/A' ⍝ second DOMAIN ERROR: return 'N/A'
11::∇⍎¨⍵ ⍝ first DOMAIN ERROR: maybe the argument is a vector of strings?
G×⍵[1]×⍵[2]÷⍵[3]*2 ⍝ the argument is a vector of numbers
}
⍝ Calculate gravity force between the Earth and the Sun
Gravity '1.99e30' '5.97e24' '1.50e11'
3.524119391E22
Gravity 1.99e30 5.97e24 1.50e11
3.524119391E22
Gravity 1.99e30 5.97e24 0 ⍝ trigger division by zero
N/A
External links
References
- ↑ Error Guards – Dyalog APL.
- ↑ APL Error Messages and Codes – Dyalog APL.