Tacit programming

Tacit functions apply to implicit arguments following a small set of rules. This is in contrast to the explicit use of arguments in dfns (⍺ ⍵) and tradfns (which have named arguments). Known dialects which implement trains are Dyalog APL, dzaima/APL, ngn/apl and NARS2000.

Primitives

All primitive functions are tacit. Some APLs allow primitive functions to be named.

      plus ← +
      times ← ×
      6 times 3 plus 5
48

Derived functions

Functions derived from an operator and operand are tacit.

      sum ← +/
      sum ⍳10
55

Trains

A train is a series of functions in isolation. An isolated function is either surrounded by parentheses or named. Arguments are processed by the following rules:

A 2-train is an atop:

  (g h) ⍵ ⬄ g (  h ⍵)
⍺ (g h) ⍵ ⬄ g (⍺ h ⍵)

A 3-train is a fork:

  (f g h) ⍵ ⬄ (  f ⍵) g (  h ⍵)
⍺ (f g h) ⍵ ⬄ (⍺ f ⍵) g (⍺ h ⍵)

The left tine of a fork (but not an atop) can be an array:

  (A g h) ⍵ ⬄ A g (  h ⍵)
⍺ (A g h) ⍵ ⬄ A g (⍺ h ⍵)

Examples

One of the major benefits of tacit programming is the ability to convey a short, well-defined idea as an isolated expression. This aids both human readability (semantic density) and the computer's ability to interpret code, potentially executing special code for particular idioms.

Plus and minus

      (+,-)2
2 ¯2
      1 2 3 (+,-) 4
5 6 7 ¯3 ¯2 ¯1
      (2 3⍴0) (+,-) 1
1 1 1 ¯1 ¯1 ¯1
1 1 1 ¯1 ¯1 ¯1

Arithmetic mean

      (+⌿÷≢) ⍳10       ⍝ Mean of the first ten integers
5.5
      (+⌿÷≢) 5 4⍴⍳4    ⍝ Mean of columns in a matrix
1 2 3 4

Fractions

We can convert decimal numbers to fractions. For example, we can convert $2.625$ to the improper fraction $21 \over 8$ with

      (1∧⊢,÷)2.625
21 8

Alternatively, we can convert it to the mixed fraction $2{5 \over 8}$ with A mixed fraction:

      (1∧0 1∘⊤,÷)2.625
2 5 8

Is it a palindrome?

      (⌽≡⊢)'racecar'
1
      (⌽≡⊢)'racecat'
0

Split delimited text

      ','(≠⊆⊢)'comma,delimited,text'
┌─────┬─────────┬────┐
│comma│delimited│text│
└─────┴─────────┴────┘
      ' '(≠⊆⊢)'space delimited text'
┌─────┬─────────┬────┐
│space│delimited│text│
└─────┴─────────┴────┘

Component of a vector in the direction of another vector

Sometimes a train can make an expression nicely resemble its equivalent definition in traditional mathematical notation. As an example, here is a program to compute the component of a vector a in the direction of another vector b.

${\textbf {a}}_{\textbf {b}}=({\textbf {a}}\cdot {\hat {\textbf {b}}}){\hat {\textbf {b}}}$

      Sqrt ← *∘.5              ⍝ Square root 
      Norm ← Sqrt+.×⍨          ⍝ Magnitude (norm) of numeric vector in Euclidean space
      Unit ← ÷∘Norm⍨           ⍝ Unit vector in direction of vector ⍵
      InDirOf ← (⊢×+.×)∘Unit   ⍝ Component of vector ⍺ in direction of vector ⍵
      3 5 2 InDirOf 0 0 1      ⍝ Trivial example
0 0 2

In particular, the definition of InDirOf resembles the definition in traditional mathematical notation:

Traditional notation	APL
$|{\textbf {b}}|$	(Sqrt+.×⍨) b
${\hat {\textbf {b}}}={\frac {\textbf {b}}{|{\textbf {b}}|}}$	(÷∘Norm⍨) b
${\textbf {a}}\cdot {\textbf {b}}$	a +.× b
$({\textbf {a}}\cdot {\hat {\textbf {b}}}){\hat {\textbf {b}}}$	a (⊢×+.×)∘Unit b