hellwolf / haskell-examples Goto Github PK

Ref: https://github.com/hellwolf/haskell-examples/blob/master/2023-02-24-wasm-comm/client.js

@hellwolf This example has some really useful utilities for working with strings in Haskell WASM reactors. Thank you for publishing it!

I was wondering about these two functions:

function withCStrings(strs, op) {
    const cstrs = strs.map(str => {
        const s = new TextEncoder().encode(str);
        const l = s.length + 1;
        const p = inst.exports.callocBuffer(l);
        const b = new bufferAt(p, l);
        b.set(s);
        return p;
    });
    const r = op(cstrs);
    strs.forEach(inst.exports.freeBuffer);
    return r;
}

function fromCString(cstr) {
    const s = new TextDecoder("utf8").decode(cstringBufferAt(cstr));
    inst.exports.freeBuffer(cstr);
    return s;
}

In the first function freeBuffer is called on strs which is presumed to be an array of utf-8 encoded strings.

In the second function freeBuffer is called on cstr, the CString representation of a string.

Is there a reason for the inconsistency? I would expect that freeBuffer use is correct in fromCString but this actually causes intermittent Memory Out of Bounds errors for me. Changing it to inst.exports.freeBuffer(s); so that it's operating on the utf-8 encoded string solves these but I have no idea why.