Text template processor

This text processor has very limited capabilities (unlike jinja, go or Perl templates.) The processor can

• selectively enable text blocks, as with if/else
• filter/replace text blocks, as for a loop implied by an iterator()
• text replacement, emulating variable interpolation

It is currently used to support CGI tools with text template files used to define the HTML layout.

Example

Let's look at this rather silly template example

   @TITLE@
   ;                  -- comment

   :IF_ENABLED:       -- outer
   +++ aaa
     :IF_NOT_ENABLED: -- inner
   +++ bbb
     :ELSE:           -- inner
   +++ ddd
     :END:            -- inner
   +++ eee
   :ELSE:             -- outer
   +++ bbb
   :END:              -- outer

   :FOR_ENV:
   *** @KEY@ = @VALUE@
   :END:

The directives :ELSE: and :END: are pre-defined by the template processor. For the template example, we set

• @TITLE@ to be replaced by the text "Hello World"

• :IF_ENABLED: to sort of evaluate true

• :FOR_ENV: to iterate through the environment variables defining @KEY@ and @VALUE@ to be replaced with appropriate text for each key/value pairs of environment entries.

then this should translate to something like

   Hello World

   +++ aaa
   +++ ddd
   +++ eee

   *** COLUMNS = 79
   *** DBUS_SESSION_BUS_ADDRESS = unix:path=/run/user/1000/bus
   *** DESKTOP_AUTOSTART_ID = 1054b02024a...
   *** DESKTOP_SESSION = lightdm-xsession
   *** ...

The corresponding NIM source code to configure the template text processor that produces this outcome looks like

   import
     algorithm, os, sequtils, tmplpro

   iterator envItems(p: TmplParser): TmplParser {.closure.} =
     for k in envPairs.toSeq.mapIt(it.key.string).sorted:
       var
         # Loop-local variables
         key = ("@KEY@", k)
         value = ("@VALUE@", k.getEnv.string)
       yield (p.filter, @[key, value].concat(p.vars))

   # Text template processor parser specification
   proc newSpecs(): TmplParser =
     result = (newTmplFilter(), newSeq[(string,string)]())

     # Global variable
     result.vars.add(("@TITLE@", "Hello World"))

     # Simple if/else clause
     result.filter["IF_ENABLED"] = toIfElse(true)

     # Loop clause
     result.filter["FOR_ENV"] = envItems.toIfElse(result)

Assuming that the template example text is assigned to the variable tmplPage, the translated text is produced and printed out with

   # Apply template processor
   echo tmplPage.tmplParser(newSpecs())