RBX.Lua Try Library

A library for controlling the flow of error-prone, interdependent functions. Get a built module on Roblox here.

How to use

Include the library's Try function by requiring the module.

local Try = require(TryLibrary)

Try(Function, Arguments...) → Attempt

This function attempts calling Function with the given list of Arguments..., and returns an Attempt, which :Then and :Catch methods can be chained to.

Attempt:Then(Function Callback) → Attempt

This method takes a callback of the form Variant Callback(ReturnValues...), and calls it if the attempt succeeded, with ReturnValues... being the list of values returned in the attempt. The attempt is then returned for chaining.

If the first value in ReturnValues... is an Attempt, it will be executed and the method will process its return values. This allows for chaining, such as in:

local HttpService = game:GetService('HttpService')

Try(wait, 0.1)

    -- Try hashing the time
    :Then(function (Delta, ElapsedTime)
        return Try(HttpService.GetAsync, HttpService, 'http://md5.jsontest.com/?text=' .. Delta)
    end)

    -- Try decoding the response
    :Then(function (RawResponse)
        return Try(HttpService.JSONDecode, HttpService, RawResponse)
    end)

    -- Print the decoded response data
    :Then(function (Response)
        print('Input:', Response.original, '\nMD5:', Response.md5)
    end)

Attempt:Catch([String Predicates...], Function Callback) → Attempt

This method takes a callback of the form Variant Callback(String Error, String Stack, Attempt FailedAttempt), and calls it if the attempt had an error. Errors can be optionally filtered by providing a list of patterns which the error should match, otherwise all errors are caught by the function. Once an attempt's error is caught, it will not be caught by the next chained :Catch method, unless Callback itself has an error. The attempt is then returned for chaining.

If the first returned value from the attempt is an Attempt, it will be executed and the method will process its errors.

local HttpService = game:GetService('HttpService')

Try(HttpService.GetAsync, HttpService, 'http://google.com/fakeurl')
    :Then(function (Data)
        print('Found', Data)
    end)

    -- Catch when the URL doesn't exist
    :Catch('HTTP 404', function (Error, Stack, Attempt)
        warn('Not found, error:', Error)
    end)

    -- Catch any other error
    :Catch(function (Error, Stack, Attempt)
        warn('Unknown error:', Error)
    end)

Attempt:Retry() → Attempt

This method retries the attempt if it failed, and executes any methods that were chained to it. Attempt.RetryCount is incremented each time the attempt is retried, and is reset if a retry succeeds.

You can use this method in combination with an attempt's :Catch method to retry a sequence of interdependent function calls that fail, and even limit the number of, or space out, retries. For example:

local HttpService = game:GetService('HttpService')

Try(HttpService.GetAsync, HttpService, 'http://httpstat.us/503')
    :Then(function (Data)
        print('Found', Data)
    end)

    -- Catch when the server is having issues and retry
    :Catch('HTTP 503', 'Timeout was reached', function (Error, Stack, Attempt)

        -- Limit the number of retries to 3
        if Attempt.RetryCount < 3 then

            -- Space out each retry
            local BackoffTime = Attempt.RetryCount * 3 + 3
            warn('Retrying in', BackoffTime, 'seconds...')
            wait(BackoffTime)

            -- Retry the attempt
            return Attempt:Retry()

        -- Give up if retry limit reached
        else
            warn('Failed')
        end

    end)

    -- Catch any other errors
    :Catch(function (Error, Stack, Attempt)
        warn('Unknown error:', Error)
    end)