## Strings. Higher-level operations on strings.

### Extra String Methods

@lookup pl.stringx

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.


### String Templates

@lookup pl.text

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]

### Another Style of Template

A new module is `template`, which is a version of Rici Lake's [Lua  Preprocessor](http://lua-users.org/wiki/SlightlyLessSimpleLuaPreprocessor).  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.

