Pyper

Pyper is a wide extension of the Lispy car/cdr idea.

Usage

Everybody knows Lispy functions #car, #cdr, #caar, #cdar, #cadr, #cddr... With Pyper, you have them at the hand:

require 'pyper'

[1, 2, 3].car # will return the first element, 1
[1, 2, 3].cdr # will return the remaining elements, [2, 3]

Similarly, #caar will return the first element of the first element, #cadr will return the first element of the remaining elements (that is, second element), #cddr will return the list of [3rd, 4th, ... ] elements, etc.

In Lisp, these compositions can theoretically extend ad infinitum:

caaar, caadr, cadar,... caaaar, caaadr, ... caaaaar, ..., cadaadr, ...

In effect, such character sequences form an APL-like language consisting of one-character operators 'a' and 'd', whose combination determines the overall operation. Pyper adds a few modifications and extends the idea:

1. Twin-barrel piping: Instead of just one pipeline, in which the operations are applied in sequence, Pyper has 2 parallel pipelines.

2. Greek letters τ, π, χ as method delimiters: Instead of 'c' and 'r' of car/cdr family, Pyper methods start and end with any of the characters τ, π, χ (tau, pi and chi). Their meaning is best explained by examples:

• τ...τ means single-pipe input and output,
• τ...π means single-pipe input, double-pipe output
• τ...χ means single-pipe input, double-pipe output with a swap
• π...τ means double-pipe input, single-pipe output
• . . .
• χ...χ means double-pipe input with a swap, and same for the output

Mnemonic for this is, that τ looks like a gutter (has one vertical pipe), π looks like a twin gutter (two pipes) and χ looks like two pipes crossed. Single-pipe input means, that a single object (the receiver) is fed to the pipeline 0, while pipeline 1 receives its copy. Double-pipe input means, that the receiver should respond to #size and #[], and its size should be 2. Pipelines 0 and 1 are then initialized respectively with the first and the second element (obtained by #[]). Double-pipe input with swap is the same, but the two elements are swapped.

3. Postfix order of commands: While traditional car/cdr family of methods applies the letters in the prefix order (from right to left), Pyper uses postfix order (left to right).

Example: #cdar becomes τadτ ('da' reversed to 'ad') #cadaar becomes τaadaτ ('adaa' reversed to 'aada')

4. Extended set of commands: The set of command characters, which in the traditional car/cdr method family consists only of two characters, 'a' and 'd', is greatly extended.

For example, apart from 'a', mening first, 'b' means second, and 'c' means third:

["See", "you", "later", "alligator"].τaτ    #=>    "See"
["See", "you", "later", "alligator"].τbτ    #=>    "you"
["See", "you", "later", "alligator"].τcτ    #=>    "later"

For another example, apart from 'd', meaning all except first, 'e' means all except first two, and 'f' means all except first three:

["See", "you", "later", "alligator"].τdτ #=> ["you", "later", "alligator"]
["See", "you", "later", "alligator"].τeτ #=> ["later", "alligator"]
["See", "you", "later", "alligator"].τfτ #=> ["alligator"]

These command characters can be combined just like 'a' and 'd' letters in the traditional car/cdr family - just beware of the Pyper's postfix order:

["See", "you", "later", "alligator"].τddτ #=> ["later", "alligator"]
["See", "you", "later", "alligator"].τdeτ #=> ["alligator"]
["See", "you", "later", "alligator"].τdeaτ #=> "alligator"
["See", "you", "later", "alligator"].τdeadτ #=> "lligator"
["See", "you", "later", "alligator"].τdeafτ #=> "igator"
["See", "you", "later", "alligator"].τdeafbτ #=> "g"

Allready with these few command characters (a-c, d-f, u-w, x-z, plus numbers 0-4 and 5-9), one can compose intelligent car/cdr-like methods. But there are more command characters available, representing various common Ruby methods, operators etc.

5. Method arguments are possible: Unlike the traditional car/cdr family, Pyper methods accept arguments. Regardless of the combination of the command characters, any Pyper method can accept an arbitrary number of arguments, which are [collected] into the 'args' variable, from which the methods triggered by the command characters may take their arguments as their arity requires.

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So much for the main concepts. As for the character meanings, those are defined as PostfixMachine methods of the same name (the name consists of 1 or 2 characters). At the moment, it is necessary to read the code for their documentation.