Source Code Filters

A Source Code Filter 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) Nim lists the processed code after each filter application.

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:

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>