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: $$. </div> </body>
The filter transforms this into:
proc generateHTMLPage(title, currentTab, content: string, tabs: openArray[string]): string = result = "" result.add("<head><title>" & $(title) & "</title></head>\n" & "<body>\n" & " <div id=\"menu\">\n" & " <ul>\n") for tab in items(tabs): if currentTab == tab: result.add(" <li><a id=\"selected\" \n") else: result.add(" <li><a\n") #end result.add(" href=\"" & $(tab) & ".html\">" & $(tab) & "</a></li>\n") #end result.add(" </ul>\n" & " </div>\n" & " <div id=\"content\">\n" & " " & $(content) & "\n" & " A dollar: $.\n" & " </div>\n" & "</body>\n")
Each line that does not start with the meta character (ignoring leading whitespace) is converted to a string literal that is added to result.
The substitution character introduces a Nim expression e within the string literal. e is converted to a string with the toString operation which defaults to $. For strong type checking, set toString to the empty string. e must match this PEG pattern:
e <- [a-zA-Z\128-\255][a-zA-Z0-9\128-\255_.]* / '{' x '}' x <- '{' x+ '}' / [^}]*
To produce a single substitution character it has to be doubled: $$ 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>