Progress report in the Pen programming language

@raviqqe

Progress report

keys and values built-in functions

• The keys function returns keys of a map.
• The values function returns values of a map.
• An order of keys and values are consistent among different instances of equivalent maps.
  • i.e. xs == ys where xs and ys are maps
• Note that key orders depends on their implementations because maps in Pen are hash maps.

xs = {string: number "foo": 1, "bar": 2}

keys(xs) == [string "bar", "foo"]
values(xs) == [number 2, 1]

Parallel list comprehension

• Natural extension to list comprehension for zip-ish computation
• Not related to parallel computation

[number x() + y() for x, y in xs, ys]

In Haskell

• With the ParallelListComp language extension in GHC

[x + y | x <- xs | y <- ys]

Use cases

Zipping

• Example: JSON serialization
• Removal of list comprehension with map iteratees

String'Join(
  [string
    serializeString(key()) + ":" + serializeValue(value())
    for key, value in keys(map), values(map)
  ],
  ",",
)

Use cases

Enumeration of list elements

• Example: SQL query build with placeholders
• Lists in Pen are lazy.

" where "
  + String'Join(
    [string
      field() + " = $" + Number'String(index())
      for field, index in whereFields, Number'Sequence(Number'Infinity())
    ],
    " and ",
  )

Future work

• More little language features
  • sort built-in function
• Code generator
  • For meta-programming
• Language server

Summary

• Now, Pen has parallel list comprehension for zip-ish composition of lists.