Safe Templates in Lua

Overview · Installation · Using Cosmo · API Reference · Contact Us · License

# Overview

Cosmo is a “safe templates” engine. It allows you to fill nested templates, providing many of the advantages of Turing-complete template engines, without without the downside of allowing arbitrary code in the templates.

# Installation

The current version of Cosmo is 13.01.30. This release adds Lua 5.2 support.

The previous version, 10.04.06, added, expressions to selectors $(<exp>), allowed nested [[ ]] in templates, made commas between subtemplates optional, and added a second parameter to cosmo.yield that tells Cosmo the first parameter is a literal to be included in the expansion instead of an environment.

Cosmo is installed as a rock. To install the most recent release do `luarocks install cosmo`. The Cosmo rock is in the standard repository. Installation on UNIX-based systems need the gcc toolchain.

# Using Cosmo

## Simple Form Filling

Let’s start with a simple example of filling a set of scalar values into a template: Here are a few examples of Cosmo in use:

    > values = { rank="Ace", suit="Spades" }
    > template = "$rank of $suit"
    > require("cosmo")
    > = cosmo.fill(template, values)
    Ace of Spades

Note that the template is a string that marks where values should go. We call a template variable like `$rank` a selector, and `rank` is the selector’s name. The table passed to `cosmo.fill` is the environment, and it provides the values. `$rank` will get replaced by `value.rank` (“Ace”) and `$suit` will get replaced by `value.suit` (“Spades”).

`cosmo.fill` takes two parameters at once. Cosmo also provides a “shortcut” method `f()` which takes only one parameter - the template - and returns a function that then takes the second parameter. This allows for a more compact notation:

    > = cosmo.f(template){ rank="Ace", suit="Spades" }
    Ace of Spades

A selector can be either a string or a Lua expression in parenthesis, like `$("foo" .. "bar")` is replaced by `foobar` in the template. Any variables in the expression are looked-up in the current template environment, so `$(foo)` is the same as `$foo`, and `$(rank .. suit)` would be replaced by `AceSpades` in the previous example.

## Nested Values

You aren’t restricted to scalar values; your values can be Lua tables that you can destructure using a $val|key1|key2|…|keyn syntax. For example:

    > values = { cards = { { rank = "Ace" , suit = "Spades" } } }
    > template = "$cards|1|rank of $cards|1|suit"
    > = cosmo.fill(template, values)
    Ace of Spades

As you can see above, you can either use numbers or strings as keys.

## Arguments

You can also pass arguments to a selector using the syntax $selector{ args }. The syntax for the argument list is the same as a Lua table constructor, but function definitions are not allowed, and you can use template selectors, which are looked up in the template environment.

If you pass an argument list and the selector maps to a function then Cosmo calls this function with the argument list as a table, and the selector expands to what the function returns. For example:

    > values = { message = function (arg) return arg.rank .. " of "
         .. arg.suit end }
    > template = "$message{ rank = 'Ace', suit = 'Spades' }"
    > = cosmo.fill(template, values)
    Ace of Spades

## Subtemplates

Now, suppose we have not just one card, but several. Cosmo allows us to handle this case with “subtemplates”

    > mycards = { {rank="Ace", suit="Spades"}, {rank="Queen", suit="Diamonds"}, {rank="10", suit="Hearts"} }
    > template = "$do_cards[[$rank of $suit, ]]"
    > = cosmo.fill(template, {do_cards = mycards})
    Ace of Spades, Queen of Diamonds, 10 of Hearts,

The subtemplate “$rank or $suit” could be enclosed in `[[...]]`, `[=[...]=]`, `[==[...]==]`, etc. - just like Lua’s long-quoted strings. Again, we can use the shortcut `f()`:

    > = cosmo.f(template){do_cards = mycards}
    Ace of Spades, Queen of Diamonds, 10 of Hearts,

## Subtemplates with Functions

If we don’t have a ready table that would match the template, we can set the value of `do_cards` to a function, which will yield a set of values for the subtemplate each time it’s called:

    > mycards = { {"Ace", "Spades"}, {"Queen", "Diamonds"}, {"10", "Hearts"} }
    > = cosmo.f(template){
           do_cards = function()
              for i,v in ipairs(mycards) do
                 cosmo.yield{rank=v[1], suit=v[2]}
              end
           end
        }
    Ace of Spades, Queen of Diamonds, 10 of Hearts,

You can also pass a list of arguments to this function:

    > template = "$do_cards{ true, false, true }[[$rank of $suit, ]]"
    > mycards = { {"Ace", "Spades"}, {"Queen", "Diamonds"}, {"10", "Hearts"} }
    > = cosmo.f(template){
           do_cards = function(arg)
              for i,v in ipairs(mycards) do
                 if arg[i] then cosmo.yield{rank=v[1], suit=v[2]} end
              end
           end
        }
    Ace of Spades, 10 of Hearts,

Finally, you can pass a literal to be included in the expansion instead of an environment. An example:

    > template = "$do_cards{ true, false, true }[[$rank of $suit]]"
    > mycards = { {"Ace", "Spades"}, {"Queen", "Diamonds"}, {"10", "Hearts"} }
    > = cosmo.f(template){
           do_cards = function(arg)
              local n = #mycards
              for i,v in ipairs(mycards) do
                 cosmo.yield{rank=v[1], suit=v[2]}
                 if i < n then cosmo.yield(", ", true) end
              end
           end
        }
    Ace of Spades, Queen of Diamonds, 10 of Hearts

## Alternative Subtemplates

In some cases we may want to use differente templates for different items in the list. For example, we might want to use a different template for the first and/or last item, or to use different templates for odd and even numbers. We can do this by specifying several templates, separated by a comma. In that case, cosmo will use the first template in the sequence, unless the table of values for the item contains a special field `_template`, in which case this field will be used as an index into the list of alternative templates. For instance, setting `_template` to 2 would tell cosmo to use the 2nd template for this item.

    > table.insert(mycards, {"2", "Clubs"})
    > template = "You have: $do_cards[[$rank of $suit]],[[, $rank of $suit]],[[, and $rank of $suit]]"
    > = cosmo.f(template){
           do_cards = function()
              for i,v in ipairs(mycards) do
                 local t
                 if i == #mycards then -- for the last item use the third template
                    t = 3
                 elseif i~=1 then -- use the second template for items 2...n-1
                    t = 2
                 end
                 cosmo.yield{rank=v[1], suit=v[2], _template=t}
              end
           end
        }

    You have: Ace of Spades, Queen of Diamonds, 10 of Heards, and 2 of Clubs

Note that the first item is formatted without preceeding “, ”, while the last item is preceeded by an extra “and”.

## Deeper Nesting

Templates and subtemplates can be nested to arbitrary depth. For instance, instead of formatting a set of cards, we can format a list of sets of cards:

    > players = {"John", "João"}
    > cards = {}
    > cards["John"] = mycards
    > cards["João"] = { {"Ace", "Diamonds"} }
    > template = "$do_players[[$player has $do_cards[[$rank of $suit]],
        [[, $rank of $suit]],[[, and $rank of $suit]]\n]]"
    > = cosmo.f(template){
           do_players = function()
              for i,p in ipairs(players) do
                 cosmo.yield {
                    player = p,
                    do_cards = function()
                       for i,v in ipairs(cards[p]) do
                          local t
                          if i == #mycards then
                             t = 3
                          elseif i~=1 then -- use the second template for items 2...n-1
                             t = 2
                          end
                          cosmo.yield{rank=v[1], suit=v[2], _template=t}
                       end
                    end
                 }
             end
          end
        }

    John has Ace of Spades, Queen of Diamonds, 10 of Hearts, and 2 of Clubs
    João has Ace of Diamonds

## Scope

Subtemplates can see values that were set in the higher scope:

    > template = "$do_players[[$do_cards[[$rank of $suit ($player), ]]]]"
    > = cosmo.f(template){
           do_players = function()
              for i,p in ipairs(players) do
                 cosmo.yield {
                    player = p,
                    do_cards = function()
                       for i,v in ipairs(cards[p]) do
                          cosmo.yield{rank=v[1], suit=v[2]}
                       end
                    end,
                 }
              end
           end
        }

    Ace of Spades (John), Queen of Diamonds (John), 10 of Hearts (John), 2 of Clubs (John), Ace of Diamonds (João),

Note that in this case the field “player” is set in the table of values that is passed to `do_players`, but is used one level deeper - in `do_cards`.

The scoping behavior can be overriden by setting a metatable on the environment you pass to the subtemplates.

## If

Subtemplates and arguments let you implement a more generic conditional:

    > template = "$do_players[=[$player: $n card$if{ $plural }[[s]]
                       $if{ $more, $n_more }[[(needs $2 more)]],[[(no more needed)]]\n]=]"
    > = cosmo.f(template){
           do_players = function()
              for i,p in ipairs(players) do
                 cosmo.yield {
                    player = p,
                    n = #cards[p],
                    ["if"] = function (arg)
                       if arg[1] then arg._template = 1 else arg._template = 2 end
                       cosmo.yield(arg)
                             end,
                    plural = #cards[p] > 1,
                    more = #cards[p] < 3,
                    n_more = 3 - #cards[p]
                 }
              end
           end
        }

    John: 4 cards (no more needed)
    João: 1 card (needs 2 more)

The conditional above is already present in Cosmo as `cosmo.cif`. Expressions in arguments make it more useful:

    > template = "$if{ math.fmod(x, 4) == 0, target = 'World' }[[ Hello $target! ]],
       [[ Hi $target! ]]"
    > result = cosmo.fill(template, { math = math, x = 2, ["if"] = cosmo.cif })
    > assert(result == " Hi World! ")

## Other conditionals

In some cases we want to format an set of values if some condition applies, and `cosmo.if` is not enough. This can be done with a function and a subtemplate by just replacing a for-loop with an if-block. However, since this is a common case, cosmo provides a function for it:

    > template = "$do_players[[$player: $n card$if_plural[[s]] $if_needs_more[[(needs $n more)]]\n]]"
    > = cosmo.f(template){
           do_players = function()
              for i,p in ipairs(players) do
                 cosmo.yield {
                    player = p,
                    n = #cards[p],
                    if_plural = cosmo.cond(#cards[p] > 1, {}),
                    if_needs_more = cosmo.cond(#cards[p] < 3, { n = 3 - #cards[p] })
                 }
              end
           end
        }

    John: 4 cards
    João: 1 card (needs 2 more)

Like `fill()`, `cond()` has a “shortcut” equivalent which takes only one parameter (the template) and returns a function:

    > = cosmo.f(template){
           do_players = function()
              for i,p in ipairs(players) do
                 cosmo.yield {
                    player = p,
                    n = #cards[p],
                    if_plural = cosmo.c(#cards[p] > 1){},
                    if_needs_more = cosmo.c(#cards[p] < 3){ n = 3-#cards[p] }
                 }
              end
           end
        }

    John: 4 cards
    João: 1 card (needs 2 more)

## Map and Inject

Cosmo provides two convenience functions for writing simple templates, `cosmo.map` and `cosmo.inject`. Both functions have to be passed in a template’s environment. The `cosmo.map` function yields each of its arguments in sequence, and inject yields its whole argument table. A simple example:

    > template = "<ol>\n$map{ 'Spades', 'Hearts', 'Clubs', 'Diamonds'}[[<li>$it</li>\n]]</ol>"
    > = cosmo.fill(template, { map = cosmo.map })
    <ol>
    <li>Spades</li>
    <li>Hearts</li>
    <li>Clubs</li>
    <li>Diamonds</li>
    </ol>
    > template = "$inject{ suit = 'Spades' }[[Ace of <b>$suit</b>]]"
    > = cosmo.fill(template, { inject = cosmo.inject })
    Ace of <b>Spades</b>

## Concat

Putting a delimiter between each expansion of a subtemplate is so common that Cosmo provides also provides a convenience function for it, `cosmo.concat`. This is an example:

    > template = "$concat{ cards, ', ' }[[$1 of $2]]"
    > mycards = { {"Ace", "Spades"}, {"Queen", "Diamonds"}, {"10", "Hearts"} }
    > = cosmo.f(template){
           cards = mycards,
           concat = cosmo.concat
       }
    Ace of Spades, Queen of Diamonds, 10 of Hearts

# API Reference

cosmo.compile(template, chunkname) - compiles template into a function that takes an environment and returns the filled template. Assigns chunkname as the name of this function

cosmo.fill(template, env) - same as cosmo.compile(template)(env)

cosmo.yield(env, is_literal) - fills the current subtemplate with env if is_literal is `nil` or `false` and adds it to the output stream; otherwise adds the the string env to the output stream

cosmo.cond(bool, tab) - returns a function that yields an empty environment if bool is `nil` or `false` and tab otherwise

cosmo.c(bool) - returns a function that takes a table tab and does the same thing as cosmo.cond(bool, tab)

cosmo.map{ … } - has to be used inside a template; yields each element of its argument in turn

cosmo.inject(env) - has to be used inside a template; yields its argument

cosmo.cif{ exp, … } - has to be used inside a template; yields its argument to subtemplate 2 is exp is `nil` or `false` and to subtemplate 1 otherwise

cosmo.concat{ list, [delim] } - has to be used inside a template; for each element of list yields it and, if it is not the last, yields the literal delim or “, ” is delim is `nil`

# Contact Us

For more information please contact one of the authors, Fabio Mascarenhas and Yuri Takhteyev, or write to the Sputnik Mailing List.

Comments are welcome!

# License

Cosmo is free software: it can be used for both academic and commercial purposes at absolutely no cost. There are no royalties or GNU-like “copyleft” restrictions. Cosmo qualifies as Open Source software. Its licenses are compatible with GPL. The legal details are below.

The spirit of the license is that you are free to use Cosmo for any purpose at no cost without having to ask us. The only requirement is that if you do use Cosmo, then you should give us credit by including the appropriate copyright notice somewhere in your product or its documentation.

The original Cosmo library is designed and implemented by Yuri Takhteyev, with much feedback and inspiration by André Carregal. This version is a reimplementation by Fabio Mascarenhas, with aditional features. The implementations are not derived from licensed software.

Copyright © 2008-2010 Fabio Mascarenhas. Copyright © 2007-2008 Yuri Takhteyev.

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.