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 [/=============================================================================
 
     Gaea text template language!
 
     Thanks to hkaiser, VeXocide and heller !!! on Spirit IRC
     Thanks to Daniel James, Matias Capeletto and Dave Abrahams
     for added comments and suggestions (Boost Docs List).
 
 ==============================================================================]
 
 Reserved tokens:
   [
   ]
   [u
   [def
   [decl
   [lambda
   [import
   [namespace
   [/
   [^
   ["
   [""
   ""]
   \[
   \]
 
   (the last 2 is for escaping [ and ])
 
 Comments:
 
   [/ comments. ]
   [/ nested [/ nested comments] comments. ]
   [/ balanced square braces [ and ] (i.e starts with [ and ends with ] and
      with equal number of [ and ]), are allowed inside comments. This is a
      good way to comment out code. ]
 
 Markups:
 
   [*This is a markup]
   [orderedlist [a][b][c]]
 
 Markups are always significant, except inside escapes (see ["" Escapes
 below) and except as list elements (see List). Like HTML/XML spaces
 are coalesced unless they are inside the preformatted markup
 (see [^ Preformatted below).
 
 Markup Identifiers:
 
 Identifiers consist of one or more letters, digits and extended
 characters: ! $ % & * + - . : < = > ? @ _ ` # ' that
 cannot begin a number and cannot conflict with a reserved token.
 
 Examples:
 
   This-is-an-identifier
   ?
   *
   ==
   ?WTH
 
 Simple Strings:
 
   This is a string
 
   Jack and Jill went up the hill to fetch a pail of water.
   Jack fell down and broke his crown, and Jill came tumbling after.
 
   123 is a numeric string
 
 Grouped String:
 
   ["This is grouped a string]
 
 The [" and ] delimit the extent of the string. A simple form
 of grouped strings are allowed *ONLY* as list (see List below)
 elements:
 
   [This grouped string is allowed *ONLY* as a list element]
 
 The double quote is not necessary because bare markups are not
 allowed as list elements. If you need to markup a list element,
 put them in braces:
 
   [[*A marked-up list element]]
 
 Marked up Strings:
 
   This string [*contains] a markup. '*' should better be a
   template, otherwise this is an error.
 
   [*This] string contains a markup. '*' should better be a
   template, otherwise this is an error.
 
 Lists:
 
 A list may contain one or more grouped-string or nested list, but
 not bare markups.
 
 Examples:
 
   [[a][b][c]]     [/ 1st ]
   [[a][[b][c]]]   [/ 2nd ]
 
 Lists can form linear or hierarchical data structures. 1st is a
 3-element list [a][b][c]. 2nd is a 2-element list where the first
 element is [a] and the second is a 2-element list [b][c].
 
 Formatting makes it clear:
 
   1st:
 
     [
       [a][b][c]
     ]
 
   2nd:
 
     [
       [a]
       [
         [b][c]
       ]
     ]
 
 This is an erroneous list:
 
   [[a][b][c] duh!]     [/ not a list ]
 
 There should never be "naked" list elements (those without
 braces). That one above is not a list. It is a grouped string;
 and so is this:
 
   [duh! [a][b][c]]     [/ not a list ]
 
 It is important to keep in mind that lists may not contain bare
 markups. This:
 
   [[dup x][dup y]]
 
 will *NOT* expand the 'dup' templates. If you need markups expanded
 in list elements, put them in braces:
 
   [[[dup x]][[dup y]]]
 
 Strings as lists:
 
 A string is just a special form of list with single character elements.
 
 List elements:
 
   [
     [This is a grouped string]
 
     [[*This is a markup]]
 
     [[a][b][c]]                         [/ A nested list ]
 
     [This string [*contains] a markup]
 
     [[*this] is still a string.]
 
     [[this is a single element list!]]
 
     ["[this is *not* a nested list!]]   [/ the double quote " prevents this from
                                            becoming a list (see Protect below)]
   ]
 
 Nil:
 
     []
 
 A nil can be an empty string or an empty list.
 
 Unicode Code Points:
 
   [u2018]                     [/ Generates unicode code point (hexadecimal). ]
 
 Escapes:
 
   \[                          [/ Escapes the open bracket]
   \]                          [/ Escapes the close bracket]
   [""x""]                     [/ Escapes all occurances of [ and ] in x. ]
 
 Preformatted:
 
   [^ some-text]               [/ Make spaces, tabs and newlines significant ]
 
 Protect:
 
   ["* blah]                   [/ Don't expand * regardless if * is
                                  a template (or not) ]
   ["[x][y][z]]                [/ Don't make this a list ]
 
 Protect ["* blah] is not really the same as [""* blah""]. Only the
 template * is not expanded. For example, ["* [bar]] will still expand
 bar, while [""* [bar]""] will not evaluate both * and bar.
 
 Template:
 Forward declarations:
 
   [decl [foo]]
 
   [decl [dup a]]
 
 Forward declarations are good for recursion and is always perfect
 for documentation.
 
 Definitions:
 
   [def [pi] 3.14159265]       [/ nullary template def ]
 
   [def pi 3.14159265]         [/ short for nullary template def ]
 
   [def [dup a] [a][a]]        [/ arguments are local templates that
                                  need to be expanded, e.g. [a] ]
 
   [def [cat a b] [a][b]]
 
 Template expansion:
 
   [pi]                        [/ nullary template ]
 
   [dup apple pie]             [/ unary template. argument is a string. ]
 
   [dup [apple pie]]           [/ unary template. argument is a 1-element list. ]
 
   [cat [apple][pie]]          [/ 2 arguments. argument is a 2-element list. ]
 
 Optional Named Arguments:
 
 Any argument can be made optional by preceding the formal argument
 name with the tilde '~'. Example:
 
   [decl [$ path ~width ~height]]
 
 The $ template above can be used this way:
 
   [$image.jpg [~width 200px] [~height 200px]]
 
 Optional arguments can be placed anywhere in the template expansion.
 Order does not matter:
 
   [$[~width 200px] image.jpg [~height 200px]]   [/ OK]
   [$[~height 200px] [~width 200px] image.jpg]   [/ OK]
 
 Notice too that image.jpg need not be placed inside braces. All optional
 arguments are gathered and what's left should conform to the rules for
 template expansion above; hence becomes:
 
   [$image.jpg]                [/ nullary template ]
 
 Of course you can place them all inside braces if you want:
 
   [$[image.jpg] [~width 200px] [~height 200px]]
 
 Variable args:
 
 Example:
 
   [decl [table [~title] . rows]]
 
 The dot '.' before the last formal argument signifies variable
 number of arguments. It is only allowed before the final formal
 argument. The actual arguments passed are collected in one list.
 
 Example invocation:
 
   [table [~title My First Table]
       [[Heading 1] [Heading 2] [Heading 3]]
       [[R0-C0]     [R0-C1]     [R0-C2]]
   ]
 
 Again, the list may contain one or more grouped-string or nested list.
 
 Variables:
 
 Templates above are purely functional with no sife-effects. Variables
 on the other hand allow side-effects:
 
   [var x value]               [/ declare and define a value ]
   [set x value]               [/ assign a new value to a variable ]
 
 Variables are special beasts. They are the only means for imperative
 programming in the language. They respect the same scope as templates.
 You cannot have a template and a variable with the same name. They can
 take in any value (strings, lists and even lambda templates (see
 below)). But unlike constant templates which cannot be redefined, values
 of variables are mutable.
 
 Intrinsics:
 
   [decl [head x]]             [/ Get first element from x. ]
   [decl [tail x]]             [/ Return a list or string without the first element of x. ]
   [decl [empty x]]            [/ Return 1 if x is empty else 0. ]
   [decl [at x n]]             [/ Return the nth element of x. ]
   [decl [size x]]             [/ Return size of x. ]
   [decl [append x e]]         [/ Append e to x. ]
   [decl [insert x e n]]       [/ Insert e to x at position n. ]
   [decl [reverse x]]          [/ Reverse x. ]
   [decl [join x y]]           [/ Join x and y as one longer list. ]
 
   [decl [fold x s f]]         [/ For a list x, initial state s, and binary template f,
                                  fold returns the result of the repeated application of
                                  [f e s] for each element e of list x, to the result
                                  of the previous f invocation (s if it is the first call). ]
 
 Many list operations can be implemented using fold. Yet, for the sake
 of efficiency, we provide these common operations as intrinsics.
 
   [decl [transform x f]]      [/ For a list x and template f, transform returns a new
                                  list with elements created by applying [f e] to each
                                  element e of x. ]
 
 Since strings are just special forms of lists, all templates that accept
 lists can also accept strings.
 
   [decl [load file]]          [/ Loads file. ]
   [decl [save file x]]        [/ Saves x to file. ]
 
 load processes files (i.e. markups are significant). If you want to load
 files verbatim, enclose it in the escape markup. Example:
 
   [""[load text.gia]""]
 
 Load works similarly to C #include. It is possible to load a file inside
 a template body. All the templates in the loaded file will be available
 in the scope where it is loaded.
 
   [decl [add a b]]            [/ Add a and b. ]
   [decl [subt a b]]           [/ Subract a and b. ]
   [decl [mult a b]]           [/ Multiply a and b. ]
   [decl [div a b]]            [/ Divide a and b. ]
   [decl [mod a b]]            [/ Remainder of a / b. ]
 
 All arithmetic templates above expect both arguments to be numeric strings.
 
-So far, all the templates above are purely functional with no
+
-sife-effects. We have only two intrinsic templates with side-effects:
+
   [decl [and a b]]            [/ Returns a and b (boolean logic). ]
-  [var x value]             [/ declare and define a value ]
+
-  [set x value]             [/ assign a new value to a variable ]
+
 
 Other comparisons and booleans can be synthesized from the basics above.
 For example not can be defined as:
-They can take in any value (strings, lists and even lambda templates
+
-(see below)). But unlike templates that can be defined but cannot be
+
-redefined, values of variables are mutable.
+
 ne (not-equal) can be defined as:
 
   [def [ne a b] [not [eq [a][b]]]], etc.
 
 Lambda Templates:
 
 Templates are first class and can be passed to and returned from other
 templates. For example, the transform intrinsic requires a lambda template
 for its last (f) argument.
 
 Here's an example of a lambda template:
 
   [lambda [x] [dup [x]]]
 
 Its syntax resembles a template definition.
 
 Here's a sample invocation of transform:
 
   [transform [[a][b][c]] [lambda [x] [dup [x]]]]
 
 This doubles all elements of the list:
 
   [[aa][bb][cc]]
 
 Template arguments:
 
 If the arity of the template argument is the same as the one expected,
 the argument (e.g. Transform expects its f argument to be a unary
 template. Likewise, dup is also a unary template), the template can
 be passed as-is. Example:
 
   [transform [[a][b][c]] [dup]]
 
 Hence:
 
   [dup]
 
 is a shorthand equivalent to:
 
   [lambda [x] [dup [x]]]
 
 Scopes:
 
 Templates can be placed inside a namespace. Example:
 
   [namespace ns
     [decl [foo x]]
     [decl [bar x]]
   ]
 
 The template x can be referenced in other scopes using the
 dot notation. Example:
 
   [ns.foo hello, world!]
 
 You may import names from other scopes this way:
 
   [import ns.foo]           [/ Import foo from ns ]
   [import ns.bar]           [/ Import bar from ns ]
 
 Then you can use foo and bar without qualification:
 
   [foo hello, world!]
   [bar howdy, joel!]
 
 You may also import a whole namespace:
 
   [import ns]               [/ Import all templates in ns ]
 
 All the intrinsics are declared in namespace "std". There is an
 implicit
 
   [import std]
 
 before anything else. In case of ambiguity (when template names clash),
 you can use explicit qualification. Example:
 
   [std.plus [1][2]]
 
 Standard Library:
 
 Apart from the intrinsics, additional support templates are auto-loaded
 at startup. These templates are all in namespace std.