created: 20140210141217955
modified: 20150124184229000
tags: Filters
title: Filter Syntax
type: text/vnd.tiddlywiki
list: [[Filter Expression]] [[Filter Run]] [[Filter Step]] [[Filter Parameter]] [[Filter Whitespace]]

<<.preamble """[[Filters]] follow a grammar that is presented here, using [[railroad diagrams|Railroad Diagrams]], for those who find formal syntax descriptions helpful. However, you can [[learn to write filters|Introduction to filter notation]] without needing to understand this group of tiddlers.""">>

A <<.def filter>> is a pipeline for transforming an <<.def input>> into an <<.def output>>. Both the input and the output are [[ordered sets of titles|Title Selection]] of things like tiddlers and fields.

Filters are [[expressions|Filter Expression]] constructed from smaller building blocks, called [[runs|Filter Run]] and [[steps|Filter Step]], each of which also transforms an input to an output.

A filter starts with an empty output. Its runs are processed from left to right, progressively modifying the output.

Here are details of the various building blocks involved:

<<list-links "[tag[Filter Syntax]]">>