CLI Args

Use command line arguments to read a file using the basic-cli platform.


# Run with `roc ./examples/CommandLineArgs/main.roc -- examples/CommandLineArgs/input.txt`
app "command-line-args"
    packages {
        pf: "",
    imports [
    provides [main] to pf

main : Task.Task {} I32
main =
    finalTask =
        # try to read the first command line argument
        pathArg <- Task.await readFirstArgT

        readFileToStr (Path.fromStr pathArg)

    finalResult <- Task.attempt finalTask

    when finalResult is
        Err ZeroArgsGiven ->
            {} <- Stderr.line "Error ZeroArgsGiven:\n\tI expected one argument, but I got none.\n\tRun the app like this: `roc command-line-args.roc -- path/to/input.txt`" |> Task.await

            Task.err 1 # 1 is an exit code to indicate failure

        Err (ReadFileErr errMsg) ->
            indentedErrMsg = indentLines errMsg
            {} <- Stderr.line "Error ReadFileErr:\n\(indentedErrMsg)" |> Task.await

            Task.err 1 # 1 is an exit code to indicate failure

        Ok fileContentStr ->
            Stdout.line "file content: \(fileContentStr)"

# Task to read the first CLI arg (= Str)
readFirstArgT : Task.Task Str [ZeroArgsGiven]_
readFirstArgT =
    # read all command line arguments
    args <- Arg.list |> Task.await

    # get the second argument, the first is the executable's path
    List.get args 1 |> Result.mapErr (\_ -> ZeroArgsGiven) |> Task.fromResult

# reads a file and puts all lines in one Str
readFileToStr : Path.Path -> Task.Task Str [ReadFileErr Str]_
readFileToStr = \path ->
    |> File.readUtf8
    # Make a nice error message 
    |> Task.mapErr
        (\fileReadErr ->
            pathStr = Path.display path
            # TODO use FileReadErrToErrMsg when it is implemented:
            when fileReadErr is
                FileReadErr _ readErr ->
                    readErrStr = File.readErrToStr readErr
                    ReadFileErr "Failed to read file at:\n\t\(pathStr)\n\(readErrStr)"

                FileReadUtf8Err _ _ ->
                    ReadFileErr "I could not read the file:\n\t\(pathStr)\nIt contains charcaters that are not valid UTF-8:\n\t- Check if the file is encoded using a different format and convert it to UTF-8.\n\t- Check if the file is corrupted.\n\t- Find the characters that are not valid UTF-8 and fix or remove them."

# indent all lines in a Str with a single tab
indentLines: Str -> Str
indentLines = \inputStr ->
    Str.split inputStr "\n"
    |> (\line -> Str.concat "\t" line)
    |> Str.joinWith "\n"


Run this from the directory that has main.roc in it:

$ roc run -- input.txt
file content: 42