Source Code Filters

A Source Code Filter (SCF) transforms the input character stream to an in-memory output stream before parsing. A filter can be used to provide templating systems or preprocessors.

To use a filter for a source file the #? notation is used:

#? stdtmpl(subsChar = '$', metaChar = '#')
#proc generateXML(name, age: string): string =
#  result = ""
<xml>
  <name>$name</name>
  <age>$age</age>
</xml>

As the example shows, passing arguments to a filter can be done just like an ordinary procedure call with named or positional arguments. The available parameters depend on the invoked filter. Before version 0.12.0 of the language #! was used instead of #?.

Hint: With --hint[codeBegin]:on or --verbosity:2 (or higher) while compiling or nim check, Nim lists the processed code after each filter application.

Usage

First, put your SCF code in a separate file with filters specified in the first line. Note: You can name your SCF file with any file extension you want, but the conventional extension is .tmpl.

If we use generateXML code shown above and call the SCF file xmlGen.tmpl In your main.nim:

include "xmlGen.tmpl"

echo generateXML("John Smith","42")

Pipe operator

Filters can be combined with the | pipe operator:

#? strip(startswith="<") | stdtmpl
#proc generateXML(name, age: string): string =
#  result = ""
<xml>
  <name>$name</name>
  <age>$age</age>
</xml>

Available filters

Replace filter

The replace filter replaces substrings in each line.

Parameters and their defaults:

sub: string = ""
the substring that is searched for
by: string = ""
the string the substring is replaced with

Strip filter

The strip filter simply removes leading and trailing whitespace from each line.

Parameters and their defaults:

startswith: string = ""
strip only the lines that start with startswith (ignoring leading whitespace). If empty every line is stripped.
leading: bool = true
strip leading whitespace
trailing: bool = true
strip trailing whitespace

StdTmpl filter

The stdtmpl filter provides a simple templating engine for Nim. The filter uses a line based parser: Lines prefixed with a meta character (default: #) contain Nim code, other lines are verbatim. Because indentation-based parsing is not suited for a templating engine, control flow statements need end X delimiters.

Parameters and their defaults:

metaChar: char = '#'
prefix for a line that contains Nim code
subsChar: char = '$'
prefix for a Nim expression within a template line
conc: string = " & "
the operation for concatenation
emit: string = "result.add"
the operation to emit a string literal
toString: string = "$"
the operation that is applied to each expression

Example:

#? stdtmpl | standard
#proc generateHTMLPage(title, currentTab, content: string,
#                      tabs: openArray[string]): string =
#  result = ""
<head><title>$title</title></head>
<body>
  <div id="menu">
    <ul>
  #for tab in items(tabs):
    #if currentTab == tab:
    <li><a id="selected"
    #else:
    <li><a
    #end if
    href="${tab}.html">$tab</a></li>
  #end for
    </ul>
  </div>
  <div id="content">
    $content
    A dollar: $$.
  </div>
</body>

The filter transforms this into:

proc generateHTMLPage(title, currentTab, content: string,
                      tabs: openArray[string]): string =
  result = ""
  result.add("<head><title>" & $(title) & "</title></head>\n" &
    "<body>\n" &
    "  <div id=\"menu\">\n" &
    "    <ul>\n")
  for tab in items(tabs):
    if currentTab == tab:
      result.add("    <li><a id=\"selected\" \n")
    else:
      result.add("    <li><a\n")
    #end
    result.add("    href=\"" & $(tab) & ".html\">" & $(tab) & "</a></li>\n")
  #end
  result.add("    </ul>\n" &
    "  </div>\n" &
    "  <div id=\"content\">\n" &
    "    " & $(content) & "\n" &
    "    A dollar: $.\n" &
    "  </div>\n" &
    "</body>\n")

Each line that does not start with the meta character (ignoring leading whitespace) is converted to a string literal that is added to result.

The substitution character introduces a Nim expression e within the string literal. e is converted to a string with the toString operation which defaults to $. For strong type checking, set toString to the empty string. e must match this PEG pattern:

e <- [a-zA-Z\128-\255][a-zA-Z0-9\128-\255_.]* / '{' x '}'
x <- '{' x+ '}' / [^}]*

To produce a single substitution character it has to be doubled: $$ produces $.

The template engine is quite flexible. It is easy to produce a procedure that writes the template code directly to a file:

#? stdtmpl(emit="f.write") | standard
#proc writeHTMLPage(f: File, title, currentTab, content: string,
#                   tabs: openArray[string]) =
<head><title>$title</title></head>
<body>
  <div id="menu">
    <ul>
  #for tab in items(tabs):
    #if currentTab == tab:
    <li><a id="selected"
    #else:
    <li><a
    #end if
    href="${tab}.html" title = "$title - $tab">$tab</a></li>
  #end for
    </ul>
  </div>
  <div id="content">
    $content
    A dollar: $$.
  </div>
</body>