# 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 a monadic operator and an operand, or from a dyadic operator and two operands are tacit functions:

```
Sum ← +/
Sum ⍳10
55
Dot ← +.×
3 1 4 dot 2 7 1
17
```

## Derived operators

A dyadic operator with its right operand forms a tacit monadic operator:

```
1(+⍣2)10
12
Twice ← ⍣2
1 +Twice 10
12
```

## Trains

A train is a series of functions in isolation. An isolated function is either surrounded by parentheses or named. Below, `⍺`

and `⍵`

refer to the arguments of the train. `f`

, `g`

, and `h`

are function (which themselves can be tacit or not), and `A`

is an array. The 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 can be an array:

```
(A g h)
``` |
```
A g ( h ⍵)
``` | |

```
⍺ (A g h) ⍵
``` |
```
A g (⍺ h ⍵)
``` |

Only dzaima/APL allows `(A h)`

, which it treats as `A∘h`

.^{[1]}

## 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 ¯2
5 (+,-) 2 ⍝ 5±2
7 3
```

### 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 to the improper fraction with

```
(1∧⊢,÷)2.625
21 8
```

Alternatively, we can convert it to the mixed fraction 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 in the direction of another vector :

```
Root ← *∘÷⍨ ⍝ Nth root
Norm ← 2 Root +.×⍨ ⍝ 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
```

For a more parallel comparison of the notations, see the comparison with traditional mathematics.

## External links

## Tutorials

- Dyalog: version 14.0 release notes
- APL Cultivation: Transcribing to and reading trains
- APLtrainer: How to read trains in Dyalog APL code (video)
- APLtrainer: Function trains in APL (video)
- Dyalog Webinar: Train Spotting in Dyalog APL (video)
- Dyalog '13: Train Spotting in Version 14.0 (video)

## Documentation

## References

- ↑ dzaima/APL: Differences from Dyalog APL. Retrieved 09 Jan 2020.