188 lines
5.3 KiB
Plaintext
188 lines
5.3 KiB
Plaintext
ell
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===
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:compact-option:
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'ell' is a modified subset of Scheme with added syntax sugar, incorporating
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ideas from Perl, Tcl and Bourne shell. The goal was to conceive a programming
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language implementable with as little code as possible while still being
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reasonably comfortable to use.
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This package is an implementation of said language, meant to be self-contained,
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portable and reusable. Performance is specifically not an intent.
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The project is currently in a "proof of concept" stage with many useful data
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operations missing but I believe it won't be a problem to implement them as
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needed for anyone interested.
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Syntax
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------
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Owing to its Scheme heritage, 'ell' is homoiconic, that is a program can be
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directly expressed using the language's data types. There are only two of
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those: the list and the string. Any numerical conversions are made on an
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as-needed basis. Similarly, strings act like atoms/symbols when executed.
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The parser, however, does a bunch of transformations:
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* `[a b c]` makes a call to `(list a b c)`;
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* `@var` is a shorthand for `(set var)`;
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* `{ code }` is the most complex one. Each line within the curly braces is
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wrapped in parentheses, and the resulting list is quoted, so that it doesn't
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execute immediately.
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As an example, consider the following snippet:
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print (if { eq? @var foo } {
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quote 'Hello world\n'
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} else {
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quote 'Error\n'
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})
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which gets expanded to the following:
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((print (if (quote ((eq? (set var) foo)))
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(quote ((quote 'Hello world\n')))
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else
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(quote ((quote 'Error\n'))))))
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Observe that the whole program is enclosed in an implicit pair of `{}` and that
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`quote` is a very powerful special form which can replace many others if needed.
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For a slightly more realistic example have a look at 'greet.ell'.
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Runtime
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-------
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All variables are put in a single global namespace with no further scoping.
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Arguments to a block (which is a list of lists) must be assigned to variables
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first using the `arg` special form, and that must happen before they get
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overriden by execution of a different block.
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When evaluating a command, the first argument is typically a string with its
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name and it is resolved as if `set` was called on it.
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The last expression in a block is the return value.
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Special Forms
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-------------
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`quote [<arg>]...`
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Returns the arguments without any evaluation.
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`arg [<name>]...`
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Assigns arguments to the current call in order to given names.
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Names for which there are no values left are set to `[]`.
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Standard library
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----------------
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The standard library interprets the empty list and the empty string as false
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values, everything else is taken as true.
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`set <name> [<value>]`
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Retrieve or set a named variable. The syntax sugar for retrieval is `@`.
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`list [<item>]...`
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Return a list made of given arguments. The syntax sugar for lists is `[]`.
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`if <cond> <body> [elif <cond> <body>]... [else <body>]`
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Conditional evaluation, strings evaluate to themselves.
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`for <list> <body>`
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Run the body for each element.
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`break`
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Abort the running loop.
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`map <list> <body>`
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Transform each element with the given function.
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`filter <list> <body>`
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Return a new list consisting of matching elements only.
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`.. [<string>]...`
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Concatenate strings.
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`print [<item>]...`
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Print all items in sequence--strings directly, lists as source code.
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`system <command>`
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Run a system command and return its return value.
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`parse <program>`
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Parse a program into a list of lists.
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`try <body> <handler>`
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Execute the body and pass any error to the handler instead of propagating it.
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`throw <message>`
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Throw an error. Messages starting on an underscore don't generate backtraces.
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`not <value>`
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Return a boolean with the opposite truthiness.
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`and [<body>]...`, `or [<body>]...`
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Short-circuit evaluation, trying to return whatever the bodies result in.
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`+`, `-`, `*`, `/`
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Arithmetic operations on floating point numbers.
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`=`, `<>`, `<`, `>`, `<=`, `>=`
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Arithmetic comparisons on floating point numbers.
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`eq?`, `ne?`, `lt?`, `gt?`, `le?`, `ge?`
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Simple string comparisons.
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Building and Running
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--------------------
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By default, running `make' will only build the interpreter:
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$ make
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$ ./interpreter greet.ell
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Install development packages for GNU Readline to get a REPL for toying around:
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$ make repl
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$ ./repl
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Possible Ways of Complicating
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-----------------------------
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* variable scoping: the simplest is to set values in the nearest scope they
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can be found in but make `arg` an exception to that, just like in AWK
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* reference counting: currently all values are always copied as needed, which
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is good enough for all imaginable use cases, simpler and less error-prone
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Contributing and Support
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------------------------
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Use this project's GitHub to report any bugs, request features, or submit pull
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requests. If you want to discuss this project, or maybe just hang out with
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the developer, feel free to join me at irc://irc.janouch.name, channel #dev.
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Bitcoin donations: 12r5uEWEgcHC46xd64tt3hHt9EUvYYDHe9
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License
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-------
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'ell' is written by Přemysl Janouch <p.janouch@gmail.com>.
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You may use the software under the terms of the ISC license, the text of which
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is included within the package, or, at your option, you may relicense the work
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under the MIT or the Modified BSD License, as listed at the following site:
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http://www.gnu.org/licenses/license-list.html
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