lua-primer.md

Lua Primer

Once you've finished reading the tutorial, you may be wondering about the relationship between Fennel and Lua. If you have never programmed in Lua before, don't fear! It is one of the simplest programming languages ever. It's possible to learn Fennel without writing any Lua code, but for certain concepts there's no substitute for the Lua documentation.

The book Programming in Lua is a great introduction. The first edition is available for free online and is still relevant, other than the section on modules. However, it's rather long, so if you have programmed before in other languages, you get going more quickly by focusing on specific areas where Lua is substantially different from other languages:

Lua's types include:

• nil: represents nothing, treated like false in conditionals
• booleans: true and false
• numbers: double-precision floating point only until integers added in 5.3
• strings: immutable, may contain arbitrary binary data
• tables: the only data structure
• coroutines: a mechanism for pre-emptive multitasking
• userdata: representing types that come from C code

Of these, tables are by far the most complex as well as being the most different from what you may be used to in other languages. The most important consideration is that tables are used for both sequential data (aka lists, vectors, or arrays) as well as associative data (aka maps, dictionaries, or hashes). The same table can be used in both roles; whether a table is sequential or associative is not an inherent property of the table itself but determined by how a given piece of code interacts with the table. Iterating over a table with ipairs will treat it as an array, while pairs will treat it as an unordered key/value map.

The Lua reference manual covers the entire language (including details of newer versions) in a more terse form which you may find more convenient when looking for specific things. The rest of this document provides a very brief overview of the standard library.

Other Lua runtimes or embedded contexts usually introduce things that aren't covered here.

Important top-level functions

• tonumber: converts its string argument to a number; takes optional base
• tostring: converts its argument to a string
• print: prints tostring of all its arguments separated by tab characters
• type: returns a string describing the type of its argument
• pcall: calls a function in protected mode so errors are not fatal
• error: halts execution and break to the nearest pcall
• assert: raises an error if a condition is nil/false, otherwise returns it
• ipairs: iterates over sequential tables
• pairs: iterates over any table, sequential or not, in undefined order
• unpack: turns a sequential table into multiple values (table.unpack in 5.2+)
• require: loads and returns a given module

Note that tostring on tables will give unsatisfactory results; simply evaluating the table in the REPL will invoke fennel.view for you, and show a human-readable view of the table (or you can invoke fennel.view explicitly in your code).

The io module

This module contains functions for operating on the filesystem. Note that directory listing is absent; you need the luafilesystem library for that.

To open a file you use io.open, which returns a file descriptor upon success, or nil and a message upon failure. This failure behavior makes it well-suited for wrapping with assert to turn failure into an error. You can call methods on the file descriptor, concluding with f:close.

(let [f (assert (io.open "path/to/file"))]
  (print (f:read)) ; reads a single line by default
  (print (f:read "*a")) ; you can read the whole file
  (f:close))

You can also call io.open with :w as its second argument to open the file in write mode and then call f:write and f:flush on the file descriptor.

The other important function in this module is the io.lines function, which returns an iterator over all the file's lines.

(each [line (io.lines "path/to/file")]
  (process-line line))

It will automatically close the file once it detects the end of the file. You can also call f:lines on a file descriptor that you got using io.open.

The table module

This contains some basic table manipulation functions. All these functions operate on sequential tables, not general key/value tables. The most important ones are described below:

The table.insert function takes a table, an optional position, and an element, and it will insert the element into the table at that position. The position defaults to being the end of the table. Similarly table.remove takes a table and an optional position, removes the element at that position, and returns it. The position defaults to the last element in the table. To remove something from a non-sequential table, simply set its key to nil.

The table.concat function returns a string that has all the elements concatenated together with an optional separator.

(let [t [1 2 3]]
  (table.insert t 2 "a") ; t is now [1 "a" 2 3]
  (table.insert t "last") ; now [1 "a" 2 3 "last"]
  (print (table.remove t)) ; prints "last"
  (table.remove t 1) ; t is now ["a" 2 3]
  (print (table.concat t ", "))) prints "a, 2, 3"

The table.sort function sorts a table in-place, as a side-effect. It takes an optional comparator function which should return true when its first argument is less than the second.

The table.unpack function returns all the elements in the table as multiple values. Note that table.unpack is just unpack in Lua 5.1.

It's not part of the table module, but the next function works with tables. It's most commonly used to detect if a table is empty, since calling it with a single table argument will return nil for empty tables. But it can also be used to step thru a table without iterators, for example:

(fn find [t x ?k]
  (match (next t ?k)
    (k x) k
    (k y) (find t x k)))

Other important modules

You can explore a module by evaluating it in the REPL to display all the functions and values it contains.

• math: all your standard math functions, trig, pseudorandom generator, etc
• string: common string operations
• os: operating system functions like exit, time, getenv, etc

What's missing

Most programming languages have a much larger standard library than Lua. You may be surprised to find that things you take for granted require third-party libraries in Lua.

Lua does not implement regular expressions but its own more limited pattern language for string.find, string.match, etc.

The lack of a string.split function surprises many people. However, the string.gmatch function used with icollect can serve to split strings into a table. Or if you just need an iterator to loop over, you can use string.gmatch directly and skip icollect.

(let [str "hello there, world"]
  (icollect [s (string.gmatch str "[^ ]+")] s))
;; -> ["hello" "there," "world"]

You can launch subprocesses with io.popen but note that you can only write to its input or read from its output; doing both cannot be done safely without some form of concurrency.

Networking requires a 3rd-party library like luasocket.

Advanced

• _G: a table of all globals
• getfenv/setfenv: access to first-class function environments in Lua 5.1; in 5.2 onward use the _ENV table instead
• getmetatable/setmetatable: metatables allow you to override the behavior of tables in flexible ways with functions of your choice
• coroutine: the coroutine module allows you to do flexible control transfer in a first-class way
• package: this module tracks and controls the loading of modules
• arg: table of command-line arguments passed to the process
• ...: arguments passed to the current function; acts as multiple values
• select: most commonly used with ... to find the number of arguments
• xpcall: acts like pcall but accepts a handler; used to get a full stack trace rather than a single line number for errors

The ... values also work at the top level of a file. They are usually used to capture command-line arguments for files run directly from the command line, but they can also pass on values from a dofile call or tell you the name of the current module in a file that's loaded from require. Note that since ... represents multiple values it is common to put it in a table to store it, unless the number of values is known ahead of time:

(local (first-arg second-arg) ...)
(local all-args [...])

Lua loading

These are used for loading Lua code. The load* functions return a "chunk" function which must be called before the code gets run, but dofile executes immediately.

• dofile
• load
• loadfile
• loadstring

Obscure

• _VERSION: the current version of Lua being used as a string
• collectgarbage: you hopefully will never need this
• debug: see the Lua manual for this module
• rawequal/rawget/rawlen/rawset: operations which bypass metatables