Strings. Higher-level operations on strings.

Extra String Methods

These are convenient borrowings from Python, as described in 3.6.1 of the Python reference, but note that indices in Lua always begin at one. stringx defines functions like isalpha and isdigit, which return true if s is only composed of letters or digits respectively. startswith and endswith are convenient ways to find substrings. (endswith works as in Python 2.5, so that f:endswith {'.bat','.exe','.cmd'} will be true for any filename which ends with these extensions.) There are justify methods and whitespace trimming functions like strip.

> stringx.import()
> ('bonzo.dog'):endswith {'.dog','.cat'}
true
> ('bonzo.txt'):endswith {'.dog','.cat'}
false
> ('bonzo.cat'):endswith {'.dog','.cat'}
true
> (' stuff'):ljust(20,'+')
'++++++++++++++ stuff'
> ('  stuff '):lstrip()
'stuff '
> ('  stuff '):rstrip()
'  stuff'
> ('  stuff '):strip()
'stuff'
> for s in ('one\ntwo\nthree\n'):lines() do print(s) end
one
two
three

Most of these can be fairly easily implemented using the Lua string library, which is more general and powerful. But they are convenient operations to have easily at hand. Note that can be injected into the string table if you use stringx.import, but a simple alias like local stringx = require 'pl.stringx' is preferrable. This is the recommended practice when writing modules for consumption by other people, since it is bad manners to change the global state of the rest of the system. Magic may be used for convenience, but there is always a price.

String Templates

Another borrowing from Python, string templates allow you to substitute values looked up in a table:

local Template = require ('pl.text').Template
t = Template('${here} is the $answer')
print(t:substitute {here = 'Lua', answer = 'best'})
==>
Lua is the best

‘$ variables’ can optionally have curly braces; this form is useful if you are glueing text together to make variables, e.g ${prefix}_name_${postfix}. The substitute method will throw an error if a $ variable is not found in the table, and the safe_substitute method will not.

The Lua implementation has an extra method, indent_substitute which is very useful for inserting blocks of text, because it adjusts indentation. Consider this example:

-- testtemplate.lua
local Template = require ('pl.text').Template

t = Template [[
    for i = 1,#$t do
        $body
    end
]]

body = Template [[
local row = $t[i]
for j = 1,#row do
    fun(row[j])
end
]]

print(t:indent_substitute {body=body,t='tbl'})

And the output is:

for i = 1,#tbl do
    local row = tbl[i]
    for j = 1,#row do
        fun(row[j])
    end
end

indent_substitute can substitute templates, and in which case they themselves will be substituted using the given table. So in this case, $t was substituted twice.

pl.text also has a number of useful functions like dedent, which strips all the initial indentation from a multiline string. As in Python, this is useful for preprocessing multiline strings if you like indenting them with your code. The function wrap is passed a long string (a paragraph) and returns a list of lines that fit into a desired line width. As an extension, there is also indent for indenting multiline strings.

New in Penlight with the 0.9 series is text.format_operator. Calling this enables Python-style string formating using the modulo operator %:

> text.format_operator()
> = '%s[%d]' % {'dog',1}
dog[1]

So in its simplest form it saves the typing involved with string.format; it will also expand $ variables using named fields:

> = '$animal[$num]' % {animal='dog',num=1}
dog[1]

As with stringx.import you have to do this explicitly, since all strings share the same metatable. But in your own scripts you can feel free to do this.

Another Style of Template

A new module is template, which is a version of Rici Lake’s Lua Preprocessor. This allows you to mix Lua code with your templates in a straightforward way. There are only two rules:

  • Lines begining with # are Lua
  • Otherwise, anything inside $() is a Lua expression.

So a template generating an HTML list would look like this:

<ul>
# for i,val in ipairs(T) do
<li>$(i) = $(val:upper())</li>
# end
</ul>

Assume the text is inside tmpl, then the template can be expanded using:

local template = require 'pl.template'
res = template.substitute(tmpl,{T = {'one','two','three'}})

and we get

<ul>
<li>1 = ONE</li>
<li>2 = TWO</li>
<li>3 = THREE</li>
</ul>

There is a single function, template.substitute which is passed a template string and an environment table. This table may contain some special fields, like \parent which can be set to a table representing a ‘fallback’ environment in case a symbol was not found. \brackets is usually ‘()’ and \_escape is usually ‘#’ but it’s sometimes necessary to redefine these if the defaults interfere with the target language – for instance, $(V) has another meaning in Make, and # means a preprocessor line in C/C++.

Finally, if something goes wrong, passing _debug will cause the intermediate Lua code to be dumped if there’s a problem.

Here is a C code generation example; something that could easily be extended to be a minimal Lua extension skeleton generator.

local subst = require 'pl.template'.substitute

local templ = [[
#include <lua.h>
#include <lauxlib.h>
#include <lualib.h>

> for _,f in ipairs(mod) do
static int l_$(f.name) (lua_State *L) {

}
> end

static const luaL_reg $(mod.name)[] = {
> for _,f in ipairs(mod) do
    {"$(f.name)",l_$(f.name)},
> end
    {NULL,NULL}
};

int luaopen_$(mod.name) {
   luaL_register (L, "$(mod.name)", $(mod.name));
    return 1;
}
]]

print(subst(templ,{
    _escape = '>',
    ipairs = ipairs,
    mod = {
        name = 'baggins';
        {name='frodo'},
        {name='bilbo'}
    }
}))

File-style I/O on Strings

pl.stringio provides just three functions; stringio.open is passed a string, and returns a file-like object for reading. It supports a read method, which takes the same arguments as standard file objects:

> f = stringio.open 'first line\n10 20 30\n'
> = f:read()
first line
> = f:read('*n','*n','*n')
10    20    30

lines and seek are also supported.

stringio.lines is a useful short-cut for iterating over all the lines in a string.

stringio.create creates a writeable file-like object. You then use write to this stream, and finally extract the builded string using value. This ‘string builder’ pattern is useful for efficiently creating large strings.

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