Dfn
- "dfns" redirects here. For the workspace by Scholes, see dfns workspace.
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.
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
Tutorials
- APL Cultivation: Diving Into Functions in APL
- Mastering Dyalog: page 174
Documentation
References
- ↑ Error Guards – Dyalog APL.
- ↑ APL Error Messages and Codes – Dyalog APL.