Strings

String code checks in strict mode and is proven pure, but it stays on CPython, and the compiler says so. String methods are fully typed (name.split(" ") is a list[str], part[0].upper() a str). A Python string has no native representation, though, so these functions run as ordinary Python on every path.

Run it

python  strings.ppy
ppy     strings.ppy
ppy run strings.ppy

What it prints

python strings.ppy, ppy strings.ppy, ppy run strings.ppy

ALK
True False
[('a', 3), ('b', 2), ('c', 1)]

Typed, checked, not lowered

@ppy.pure
def is_palindrome(text: str) -> bool:
    cleaned: str = text.lower().replace(" ", "")
    left: int = 0
    right: int = len(cleaned) - 1
    while left < right:
        if cleaned[left] != cleaned[right]:
            return False
        left += 1
        right -= 1
    return True

The loop is native-shaped, but cleaned[left] indexes a str. The checker proves the function pure, and it stays on the Python side. ppy explain strings.ppy:is_palindrome reports the first construct that blocked lowering.

Fast text goes through bytes

Text that needs to be fast goes through a byte buffer instead. Buffer[ppy.u8] is one byte per character and lowers, which is how substring search scans four million characters natively.

Where the code comes from

strings.ppy is hand-written; there is no .py source and no conversion step.

Read on: Reading input · Native lowering · Algorithms: substring search

19_strings/strings.ppy

import ppy


@ppy.pure
def initials(name: str) -> str:
    out: str = ""
    for part in name.split(" "):
        if len(part) > 0:
            out += part[0].upper()
    return out


@ppy.pure
def is_palindrome(text: str) -> bool:
    cleaned: str = text.lower().replace(" ", "")
    left: int = 0
    right: int = len(cleaned) - 1
    while left < right:
        if cleaned[left] != cleaned[right]:
            return False
        left += 1
        right -= 1
    return True


@ppy.pure
def word_count(text: str) -> dict[str, int]:
    counts: dict[str, int] = {}
    for word in text.lower().split(" "):
        counts[word] = counts.get(word, 0) + 1
    return counts


def main() -> None:
    print(initials("ada lovelace king"))
    print(is_palindrome("A man a plan a canal Panama"), is_palindrome("nope"))
    print(sorted(word_count("a b a c b a").items()))


main()

Source: examples/19_strings.