callbacks.js

// The well-known callback pattern
fs.mkdir("some-path", function(err) {
    fs.open("some-path/file", "w", function(err, fd) {
        var buffer = new Buffer("hello world")
        fs.write(fd, buffer, 0, buffer.length, null, function(err, bytes) {
            console.log("Wrote " + bytes + " bytes")
        })
    })
})

// What if we changed all the APIS to return a function that takes the callback
// It's the exact same as node core except we have a return value of a callback
function mkdir(uri) {
    return function (cb) {
        fs.mkdir(uri, cb)
    }
}

function open(uri, flags) {
    return function (cb) {
        fs.open(uri, flags, cb)
    }
}

function write(fd, buffer, offset, length, position) {
    return function (cb) {
        fs.write(fd, buffer, offset, length, position, cb)
    }
}

// Nothing is stopping you from doing the same thing with partial functions.
// Slightly different syntax, but the same pattern. Nothing major
// new to learn here.
mkdir("some-path")(function (err) {
    open("some-path/file", "w")(function (err, fd) {
        var buffer = new Buffer("hello world")
        write(fd, buffer, 0, buffer.length, null)(function (err, bytes) {
            console.log("Wrote " + bytes + " bytes")
        })
    })
})

// Want pipeable sugar. Sure that's easy.
// We introduce the notion of a duplex callback. It's a callback that takes
// a callback and returns a callback!
function pipeable(future) {
    future.pipe = function (duplex) {
        return pipeable(duplex(future))
    }
    return future
}

pipeable(mkdir("some-path"))
    // Of course we don't have the file, that would be magic!
    // Instead we have a callback that we can call. So here lets
    // Return a callback invoke the file callback, then open the fd and 
    // pass it to the callback
    .pipe(function (file) {
        return function future(callback) {
            file(function (err) {
                open("some-path/file", "w")(callback)
            })
        }
    })
    .pipe(function (fd) {
        return function future(callback) {
            fd(function (err, fd) {
                var buffer = new Buffer("hello world")
                write(fd, buffer, 0, buffer.length, null)(callback)
            })
        }
    })
    (function (err, bytes) {
        console.log("Wrote " + bytes + " bytes")
    })

// Wow that's noisy! But it's re-occuring everywhere. Can we write a function
// for this?
function map(lambda) {
    return function (source) {
        return function future(callback) {
            source(function (err, value) {
                lambda(err, value)(callback)
            })
        }
    }
}

pipeable(mkdir("some-path"))
    .pipe(map(function (err) {
        return open("some-path/file", "w")
    }))
    .pipe(map(function (err, fd) {
        var buffer = new Buffer("hello world")
        return write(fd, buffer, 0, buffer.length, null)
    }))
    // this is still a bit weird!
    (function (err, bytes) {
        console.log("Wrote " + bytes + " bytes")
    })

// let's update pipeable to support half duplex callbacks. That is 
// a callback that takes a callback and doesn't return one
function pipeable(future) {
    if (!future) return
    future.pipe = function (duplex) {
        return pipeable(duplex(future))
    }
    return future
}

pipeable(mkdir("some-path"))
    .pipe(map(function (err) {
        return open("some-path/file", "w")
    }))
    .pipe(map(function (err, fd) {
        var buffer = new Buffer("hello world")
        return write(fd, buffer, 0, buffer.length, null)
    }))
    .pipe(function (req) {
        // not quite optimum!!
        req(function (err, bytes) {
            console.log("Wrote " + bytes + " bytes")
        })
    })

// Let's create a little sink function for this
function sink(callback) {
    return function future(source) {
        source(callback)
    }
}

pipeable(mkdir("some-path"))
    .pipe(map(function (err) {
        return open("some-path/file", "w")
    }))
    .pipe(map(function (err, fd) {
        var buffer = new Buffer("hello world")
        return write(fd, buffer, 0, buffer.length, null)
    }))
    .pipe(sink(function (err, bytes) {
        console.log("Wrote " + bytes + " bytes")
    })

// BUT WHAT ABOUT ERROR HANDLING >:(
// Actually that's not hard either. Just have map pass on the
// err to the callback instead of to the lambda.
function map(lambda) {
    return function (source) {
        return function future(callback) {
            source(function (err, value) {
                if (err) {
                    return callback(err)
                }

                lambda(value)(callback)
            })
        }
    }
}

// Now just drop the errors everywhere. They will bubble up to
// the last item in the chain by default, which is the sink
pipeable(mkdir("some-path"))
    .pipe(map(function () {
        return open("some-path/file", "w")
    }))
    .pipe(map(function (fd) {
        var buffer = new Buffer("hello world")
        return write(fd, buffer, 0, buffer.length, null)
    }))
    .pipe(sink(function (err, bytes) {
        console.log("Wrote " + bytes + " bytes")
    })
    
// Now the most important part of this is that this is STILL callbacks
// Our fancy pants chaining sugar & higher order functions are 100%
// compatible with everything else in node core & npm.
function readAndWriteFile(folder, file, callback) {
    pipeable(mkdir(folder))
        .pipe(map(function () {
            return open(path.join(folder, file), "w")
        }))
        .pipe(map(function (fd) {
            var buffer = new Buffer("hello world")
            return write(fd, buffer, 0, buffer.length, null)
        }))
        .pipe(sink(callback)
}

// Or maybe let's get rid of that pipeable chaining sugar.
function readAndWriteFile(folder, file, callback) {
    var dir = mkdir(folder)
    var fd = map(function (directory) {
        return open(path.join(folder, file), "w")
    })(dir)
    map(function (fd) {
        var buffer = new Buffer("hello world")
        return write(fd, buffer, 0, buffer.length, null)
    })(fd)(callback)
}

I think if you expose classic callbacks in your module, it doesn't really matter if you use something else internally,
because your user still gets to decide wether they want to promisify(module) or continuify(module)