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Specify argument in which an error happened
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README.adoc

ell

ell is a modified subset of Scheme with added syntax sugar, incorporating ideas from Perl, Tcl and Bourne shell. The goal was to conceive a programming language implementable with as little code as possible while still being reasonably comfortable to use.

This package is an implementation of said language, meant to be self-contained, portable and reusable. Performance is specifically not an intent.

The project is currently in a "working proof of concept" stage.

Syntax

Owing to its Scheme heritage, ell is homoiconic, that is a program can be directly expressed using the language’s data types. There are only two of those: the list and the string. Any numerical conversions are made on an as-needed basis. Similarly, strings act like atoms/symbols when executed.

The parser, however, does a bunch of transformations:

  • [a b c] makes a call to (list a b c);

  • @var is a shorthand for (set var);

  • { code } is the most complex one. Each line within the curly braces is wrapped in parentheses, and the resulting list is quoted, so that it doesn’t execute immediately.

As an example, consider the following snippet:

print (if { eq? @var foo } {
    quote 'Hello world\n'
} else {
    quote 'Error\n'
})

which gets expanded to the following:

print (if (quote ((eq? (set var) foo)
            (quote quote 'Hello world\n')
            else
            (quote quote 'Error\n'))))

Observe that the whole program is enclosed in an implicit pair of {} and that quote is a very powerful special form which can replace many others if needed.

Runtime

All variables are put in a single global namespace with no further scoping. Arguments to a block (which is a list of lists) must be assigned to variables first using the arg special form, and that must happen before they get overriden by execution of a different block.

When evaluating a command, the first argument is typically a string with its name and it is resolved as if set was called on it.

The last expression in a block is the return value.

Special Forms

quote <arg>

Returns the first argument.

arg <name>…​

Reassigns arguments to the current call in order to given names. This must be a special form because of the lack of variable scoping. It needs to see arguments from the outer scope.

Standard library

The standard library interprets the empty list and the empty string as false values, everything else is taken as true.

set <name> [<value>]

Retrieve or set a named variable. The syntax sugar for retrieval is @.

list

Return a list made of given arguments. The syntax sugar for lists is [].

if <cond> <body> [elif <cond> <body>]…​ [else <body>]

Conditional evaluation, strings evaluate to themselves.

for <list> <body>

Run the body for each element.

break

Abort the running loop.

map <list> <body>

Transform each element with the given function.

filter <list> <body>

Return a new list consisting of matching elements only.

.. [<string>]…​

Concatenate strings.

print [<item>]…​

Print all items in sequence—​strings directly, lists as source code.

system <command>

Run a system command and return its return value.

+, -, *, /

Arithmetic operations on floating point numbers.

=, <>, <, >, ⇐, >=

Arithmetic comparisons on floating point numbers.

eq?, ne?, lt?, gt?, le?, ge?

Simple string comparisons.

Building and Running

By default, running `make' will only build the interpreter:

$ make
$ ./interpreter greet.ell

Install development packages for GNU Readline to get a REPL for toying around:

$ make repl
$ ./repl

Contributing and Support

Use this project’s GitHub to report any bugs, request features, or submit pull requests. If you want to discuss this project, or maybe just hang out with the developer, feel free to join me at irc://irc.janouch.name, channel #dev.

Bitcoin donations: 12r5uEWEgcHC46xd64tt3hHt9EUvYYDHe9

License

ell is written by Přemysl Janouch <p.janouch@gmail.com>.

You may use the software under the terms of the ISC license, the text of which is included within the package, or, at your option, you may relicense the work under the MIT or the Modified BSD License, as listed at the following site: