# Source Code Filters

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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 `.nimf` (it used to be `.tmpl` but that was too generic, for example preventing github to recognize it as Nim source file).

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

    include "xmlGen.nimf"

    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>