Pyper

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

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.

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.