mers/README.md

# mers
Mark's error-script -- A script language with explicit error handling and type-checks that run before the script even starts.

**This is highly experimental and a lot of things might change. It lacks very basic builtins (who needs to add integers together anyways) and has a lot of bugs.**

## checks run before the script does

One of the worst things about script languages is writing a script that runs for more than 5 minutes just to crash right before printing the output because of some type-error.
With mers, this won't happen - mers runs checks and parses the script into a special data structure before running it. This ensures that types are always what they should be and all errors can be caught before the script even starts.
(mers was born out of the need for "a script language that handles errors almost as well as Rust, but with less typing")

## Type-Checked

The main feature of this language is the type-checking. Here are some common approaches that you might be familiar with:
- Asssign a type or a Trait/Interface/Base Class/... to each variable (most statically typed languages - Java, C#, and Rust)
- Just don't - what could go wrong? (please stop doing this - JavaScript and most Scripts/Shells)

While the OOP approach is safe and won't cause weird crashes, it requires classes, interfaces or enums, which can be a lot to type for a simple script.

The JavaScript approach requires way less typing, but is prone to crashing

In mers, each variable has a type. However, this type is actually a list of all types the variable can have. This means
- v = "this is a string"
  + s will always be a string. The type-checker knows this.
- v = if some_condition { "Hello, user!" } else { -1 }
  + v could be a string or an integer. The type checker knows this, too, showing its type as [String, Int].
- v = thread( () "This is an anonymous function returning a string" )
  + Here, we use thread to run an anonymous function in the background. v will have the type Thread(String). If we do r = v.await(), r will be a String. The type-checker knows this.

To ensure that variables with more than one possible type won't cause issues, **every possible type has to be handeled**. This can, for example, be achieved using to_string(), which accepts arguments of types String, Int, Float, Tuple, or List.
If a variable could be a String, Int or Float and should be multiplied with another Float, the type-checker will complain that the String case isn't handeled because the mul() function doesn't accept String arguments.
To distinguish between different types, a *switch* statement has to be used. For pattern-matching, *match* statements can be used.

## Error handling

Using Rust has dropped the number of runtime errors/exceptions in my projects significantly. The same thing should be true for mers. Just like Rust, there are no exceptions. If a function can fail, that will be visible in the return type.
In Rust, loading a file might show Result<String, io::Error> as the functions return type. In Mers, it would be String/[], where [] is an empty Tuple similar to () in Rust. The Type-Checker will refuse to assume that the function returned String, because it might have returned nothing.
We need to handle the String case explicitly, for example using *switch*. If we don't, the type-checker wont even let the script start, preventing runtime errors.

## Multithreading

"Function" is a valid type for variables in mers.
Functions have any number of arguments and can capture their environment.
They can be executed synchronously using run(f, args..) or on a different thread using thread(f, args..).
run() will return what the function returns while thread() will return a Thread value. await(thread) will wait for the thread to finish and return its value.

## Examples

### Switching on an unknown type.

    string = false
    var = if string "test string" else -1
    
    print_int = (num int) num.to_string().print()
    
    debug( var )
    
    switch! var {
        string {
            print("String:")
            print( var )
        }
        int {
            print("integer:")
            print_int.run( var )
        }
    }

using switch! forces you to cover all possible types. Try removing the string or int case and see what happens!

### Matching on user input

In this script, we ask the user to enter some text. We match on the entered text:

- If it can be parsed as an int, we save it as an int.
- If it can't be parsed as an int, but it can be parsed as a float, we save it as a float
- If it can't be parsed as an int or a float, we return an error (of type string)

We then debug-print num (try entering '12' vs '12.5' - nums type will change!)

    println("Enter some text, I will try to parse it!")
    text = read_line()

    num = match text {
        // if possible, returns an int, otherwise a float, if that's also not possible, returns an error message in form of a string
        // parse_int and parse_float return either [] (which won't match) or int/float (which will match), so they just work with the match statement.
        parse_int(text) text
        parse_float(text) text
        // eq returns a bool, which will match if it was *true*, but not if it was *false*
        text.eq("some text") "haha, very funny..."
        // default value (true will always match)
        true "number wasn't int/float!"
    }

    "Input: \"{0}\"".format(text).println()
    num.debug()

A match arm consists of two consecutive statements: The condition statement followed by the action statement.

If the condition statement matches, the action statement will be executed. The matched value from the condition statement can be found in the variable we originally matched on, so it can be used in the action statement.

**These are the rules for matching:**
- The condition statement *does not match* if it returns **false**, **[]** or an error.
- If the condition statement returns a length 1 tuple [v], it will match and the matched value will be v. The condition statement can't return any tuple except [] and [v] to avoid confusion. To return a tuple, wrap it in a length-1 tuple: [[val_1, val_2, val_3]].
- Otherwise, the condition statement will match and the matched value will be whatever it returned.

### Reading /tmp/ and filtering for files/directories

    for file fs_list("/tmp/") {
        list = file.fs_list()
        switch! list {
            [] {
                "\"{0}\" is a file".format(file).println()
            }
            [string] {
                "\"{0}\" is a directory".format(file).println()
            }
        }
    }

### Running a thread and awaiting it, passing arguments to the thread when starting it and sharing a variable because the thread's function captured it (useful for reporting progress, i.e. changing a float from 0.0 to 100.0 while downloading and using the main thread to animate a progress bar, then using .await() only when the float is set to 100 to avoid blocking)

    print( "Starting" )

    captured = "Unchanged"

    calc_function = (prog_message string) {
        sleep( 1 )
        print( prog_message )
        sleep( 3 )
        captured = "changed from a different thread"
        "this is my output"
    }

    calc_thread = thread(calc_function "progress message!")

    sleep( 2 )
    print( "Done." )
    print( captured )

    calc_thread.await().print()
    print( captured )

## Quirks

currently, f(a b c) is the same as a.f(b c). var.function(args) will use var as the function's first argument, moving all other arguments back. This removes the need for struct/class syntax. Simply declare a function scream(str string) { str.to_upper().print() } and you can now use var.scream() on all strings.