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Tacit programming: Difference between revisions
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'''Tacit programming''', also called '''[[wikipedia:Tacit_programming|point-free style]]''', refers to usage of tacit [[function]]s that are defined in terms of implicit [[argument]]s. This is in contrast to the explicit use of arguments in [[dfn]]s (< | '''Tacit programming''', also called '''[[wikipedia:Tacit_programming|point-free style]]''', refers to usage of tacit [[function]]s that are defined in terms of implicit [[argument]]s. This is in contrast to the explicit use of arguments in [[dfn]]s (<syntaxhighlight inline lang=apl>⍺ ⍵</source>) and [[tradfn]]s (which have named arguments). Some APL dialects allow to combine functions into [[train]]s following a small set of rules. This allows creating complex [[derived function]]s without specifying any arguments explicitly. | ||
Dialects which implement trains include [[Dyalog APL]], [[dzaima/APL]], [[ngn/apl]] and [[NARS2000]]. | Dialects which implement trains include [[Dyalog APL]], [[dzaima/APL]], [[ngn/apl]] and [[NARS2000]]. | ||
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== Primitives == | == Primitives == | ||
All [[primitive functions]] are tacit. Some APLs allow primitive functions to be named. | All [[primitive functions]] are tacit. Some APLs allow primitive functions to be named. | ||
< | <syntaxhighlight lang=apl> | ||
plus ← + | plus ← + | ||
times ← × | times ← × | ||
| Line 14: | Line 14: | ||
== Derived functions == | == Derived functions == | ||
Functions derived from a monadic operator and an operand, or from a dyadic operator and two operands are tacit functions: | Functions derived from a monadic operator and an operand, or from a dyadic operator and two operands are tacit functions: | ||
< | <syntaxhighlight lang=apl> | ||
Sum ← +/ | Sum ← +/ | ||
Sum ⍳10 | Sum ⍳10 | ||
| Line 25: | Line 25: | ||
== Derived operators == | == Derived operators == | ||
A dyadic operator with its right operand forms a tacit monadic operator: | A dyadic operator with its right operand forms a tacit monadic operator: | ||
< | <syntaxhighlight lang=apl> | ||
1(+⍣2)10 | 1(+⍣2)10 | ||
12 | 12 | ||
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These rules are used for 3-trains: | These rules are used for 3-trains: | ||
{| | {| | ||
|< | |<syntaxhighlight lang=apl> (f g h) ⍵</source>|| {{←→}} ||<syntaxhighlight lang=apl>( f ⍵) g ( h ⍵)</source> | ||
|- | |- | ||
|< | |<syntaxhighlight lang=apl>⍺ (f g h) ⍵</source>|| {{←→}} ||<syntaxhighlight lang=apl>(⍺ f ⍵) g (⍺ h ⍵)</source> | ||
|} | |} | ||
The ''left tine'' of a fork can be an array: | The ''left tine'' of a fork can be an array: | ||
{| | {| | ||
|< | |<syntaxhighlight lang=apl> (A g h) ⍵</source>|| {{←→}} ||<syntaxhighlight lang=apl>A g ( h ⍵)</source> | ||
|- | |- | ||
|< | |<syntaxhighlight lang=apl>⍺ (A g h) ⍵</source>|| {{←→}} ||<syntaxhighlight lang=apl>A g (⍺ h ⍵)</source> | ||
|} | |} | ||
In APL (but not [[J]]), these rules are used for 2-trains: | In APL (but not [[J]]), these rules are used for 2-trains: | ||
{| | {| | ||
|< | |<syntaxhighlight lang=apl> (g h) ⍵</source>|| {{←→}} ||<syntaxhighlight lang=apl>g ( h ⍵)</source> | ||
|- | |- | ||
|< | |<syntaxhighlight lang=apl>⍺ (g h) ⍵</source>|| {{←→}} ||<syntaxhighlight lang=apl>g (⍺ h ⍵)</source> | ||
|} | |} | ||
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{| | {| | ||
|< | |<syntaxhighlight lang=apl>(f g h) ⍵</source>|| {{←→}} ||<syntaxhighlight lang=apl>g⍨∘f⍨∘h⍨ ⍵</source> | ||
|- | |- | ||
|< | |<syntaxhighlight lang=apl>(f g f) ⍵</source>|| {{←→}} ||<syntaxhighlight lang=apl>g⍥f⍨ ⍵</source> | ||
|- | |- | ||
|< | |<syntaxhighlight lang=apl>(⊢ g f) ⍵</source>|| {{←→}} ||<syntaxhighlight lang=apl>g∘f⍨ ⍵</source> | ||
|} | |} | ||
== Debugging == | == Debugging == | ||
In [[Dyalog APL]], analysis of trains is assisted by a [[user command]] < | In [[Dyalog APL]], analysis of trains is assisted by a [[user command]] <syntaxhighlight lang=apl inline>]Boxing on</source>. This is achieved by executing the command <syntaxhighlight lang=apl inline>]Boxing on</source> and then entering a train without any parameters. A structure of the train will be displayed. | ||
For example, the "accursed train" from the section below can be analysed like this: | For example, the "accursed train" from the section below can be analysed like this: | ||
< | <syntaxhighlight lang=apl> | ||
]Boxing on | ]Boxing on | ||
Was OFF | Was OFF | ||
| Line 89: | Line 89: | ||
Alternatively, a train can be represented in form of a tree: | Alternatively, a train can be represented in form of a tree: | ||
< | <syntaxhighlight lang=apl> | ||
]Boxing on -trains=tree | ]Boxing on -trains=tree | ||
Was ON -trains=box | Was ON -trains=box | ||
| Line 103: | Line 103: | ||
</source> | </source> | ||
Or fully parenthesised: | Or fully parenthesised: | ||
< | <syntaxhighlight lang=apl> | ||
]Boxing on -trains=parens | ]Boxing on -trains=parens | ||
Was OFF -trains=box | Was OFF -trains=box | ||
| Line 111: | Line 111: | ||
=== Conversion to dfns === | === Conversion to dfns === | ||
It can help understanding to convert a tacit function to a dfn. For many tacit functions, it is not immediately clear if the intention of the function is to be used monadically or dyadically, or even both. Such knowledge can be conveyed by comments, but sometimes it is possible to spot patterns that are exclusively monadic or dyadic: A function with a bound argument (for example < | It can help understanding to convert a tacit function to a dfn. For many tacit functions, it is not immediately clear if the intention of the function is to be used monadically or dyadically, or even both. Such knowledge can be conveyed by comments, but sometimes it is possible to spot patterns that are exclusively monadic or dyadic: A function with a bound argument (for example <syntaxhighlight lang=apl inline>+∘1</source>) can indicate a monadic function, and in some contexts, <syntaxhighlight lang=apl inline>=</source>, which can only be used dyadically, would indicate a dyadic function. The website [https://tacit.help tacit.help] provides automated translation of most tacit functions, into both monadic and dyadic, fully parenthesised dfns. | ||
== Examples == | == 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 [[idiom]]s. | 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 [[idiom]]s. | ||
=== Plus and minus === | === Plus and minus === | ||
< | <syntaxhighlight lang=apl> | ||
(+,-) 2 ⍝ ±2 | (+,-) 2 ⍝ ±2 | ||
2 ¯2 | 2 ¯2 | ||
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=== Arithmetic mean === | === Arithmetic mean === | ||
< | <syntaxhighlight lang=apl> | ||
(+⌿÷≢) ⍳10 ⍝ Mean of the first ten integers | (+⌿÷≢) ⍳10 ⍝ Mean of the first ten integers | ||
5.5 | 5.5 | ||
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=== Fractions === | === Fractions === | ||
We can convert decimal numbers to fractions. For example, we can convert <math>2.625</math> to the improper fraction <math>\tfrac{21}{8}</math> with | We can convert decimal numbers to fractions. For example, we can convert <math>2.625</math> to the improper fraction <math>\tfrac{21}{8}</math> with | ||
< | <syntaxhighlight lang=apl> | ||
(1∧⊢,÷)2.625 | (1∧⊢,÷)2.625 | ||
21 8 | 21 8 | ||
</source> | </source> | ||
Alternatively, we can convert it to the mixed fraction <math>2\tfrac{5}{8}</math> with a mixed fraction: | Alternatively, we can convert it to the mixed fraction <math>2\tfrac{5}{8}</math> with a mixed fraction: | ||
< | <syntaxhighlight lang=apl> | ||
(1∧0 1∘⊤,÷)2.625 | (1∧0 1∘⊤,÷)2.625 | ||
2 5 8 | 2 5 8 | ||
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=== Is it a palindrome? === | === Is it a palindrome? === | ||
< | <syntaxhighlight lang=apl> | ||
(⌽≡⊢)'racecar' | (⌽≡⊢)'racecar' | ||
1 | 1 | ||
| Line 152: | Line 152: | ||
=== Split delimited text === | === Split delimited text === | ||
< | <syntaxhighlight lang=apl> | ||
','(≠⊆⊢)'comma,delimited,text' | ','(≠⊆⊢)'comma,delimited,text' | ||
┌─────┬─────────┬────┐ | ┌─────┬─────────┬────┐ | ||
| Line 166: | Line 166: | ||
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 <math>\textbf{a}</math> in the direction of another vector <math>\textbf{b}</math>: | 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 <math>\textbf{a}</math> in the direction of another vector <math>\textbf{b}</math>: | ||
:::<math>\textbf{a}_\textbf{b} = (\textbf{a}\cdot\hat{\textbf{b}})\hat{\textbf{b}}</math> | :::<math>\textbf{a}_\textbf{b} = (\textbf{a}\cdot\hat{\textbf{b}})\hat{\textbf{b}}</math> | ||
< | <syntaxhighlight lang=apl> | ||
Root ← *∘÷⍨ ⍝ Nth root | Root ← *∘÷⍨ ⍝ Nth root | ||
Norm ← 2 Root +.×⍨ ⍝ Magnitude (norm) of numeric vector in Euclidean space | Norm ← 2 Root +.×⍨ ⍝ Magnitude (norm) of numeric vector in Euclidean space | ||
| Line 178: | Line 178: | ||
===The Number of the Beast=== | ===The Number of the Beast=== | ||
The following expression for computing the [[wikipedia:666 (number)|number of the Beast]] (and of [[I.P. Sharp]]'s APL-based email system, [[666 BOX]]) nicely illustrates how to read a train. | The following expression for computing the [[wikipedia:666 (number)|number of the Beast]] (and of [[I.P. Sharp]]'s APL-based email system, [[666 BOX]]) nicely illustrates how to read a train. | ||
< | <syntaxhighlight lang=apl> | ||
((+.×⍨⊢~∘.×⍨)1↓⍳)17 ⍝ Accursed train | ((+.×⍨⊢~∘.×⍨)1↓⍳)17 ⍝ Accursed train | ||
666 | 666 | ||
</source> | </source> | ||
First, < | First, <syntaxhighlight lang=apl inline>((+.×⍨⊢~∘.×)1↓⍳)</source> is supplied with only one argument <syntaxhighlight lang=apl inline>17</source> and is thus interpreted monadically. | ||
Second, < | Second, <syntaxhighlight lang=apl inline>(+.×⍨⊢~∘.×⍨)1↓⍳</source> is a 4-train: reading right-to-left, the last 3 components are interpreted as the fork <syntaxhighlight lang=apl inline>1↓⍳</source> and the 4-train is interpreted as the atop <syntaxhighlight lang=apl inline>(+.×⍨⊢~∘.×⍨)(1↓⍳)</source>. | ||
Similarly, < | Similarly, <syntaxhighlight lang=apl inline>(+.×⍨⊢~∘.×⍨)</source> is also a 4-train and interpreted as the atop <syntaxhighlight lang=apl inline>+.×⍨(⊢~∘.×⍨)</source>. | ||
Thus the accursed train is interpreted as < | Thus the accursed train is interpreted as <syntaxhighlight lang=apl inline>((+.×⍨(⊢~∘.×⍨))(1↓⍳))17</source>. Having read the train, we now evaluate it monadically. | ||
< | <syntaxhighlight lang=apl> | ||
((+.×⍨(⊢~∘.×⍨))(1↓⍳))17 ⍝ Accursed train as an atop over a fork atop a fork | ((+.×⍨(⊢~∘.×⍨))(1↓⍳))17 ⍝ Accursed train as an atop over a fork atop a fork | ||
+.×⍨(⊢~∘.×⍨)1↓⍳17 ⍝ Atop evalution | +.×⍨(⊢~∘.×⍨)1↓⍳17 ⍝ Atop evalution | ||
| Line 196: | Line 196: | ||
666 ⍝ the sum of the squares of the primes up to 17 | 666 ⍝ the sum of the squares of the primes up to 17 | ||
</source> | </source> | ||
Note that < | Note that <syntaxhighlight lang=apl inline>((⊢⍨∘.×⍨)1↓⍳)</source> is a train computing primes up to the given input. | ||
A more satisfying variation of the accursed train is the following. | A more satisfying variation of the accursed train is the following. | ||
< | <syntaxhighlight lang=apl> | ||
(⍎⊢,⍕∘≢)'((+.×⍨⊢~∘.×⍨)1↓⍳)' ⍝ Accursed train 2.0 | (⍎⊢,⍕∘≢)'((+.×⍨⊢~∘.×⍨)1↓⍳)' ⍝ Accursed train 2.0 | ||
⍎(⊢,⍕∘≢)'((+.×⍨⊢~∘.×⍨)1↓⍳)' ⍝ 4-train intepreted as an atop | ⍎(⊢,⍕∘≢)'((+.×⍨⊢~∘.×⍨)1↓⍳)' ⍝ 4-train intepreted as an atop | ||