created: 20210618133745003
from-version: 5.3.0
modified: 20230506172920710
rp-input: <<.olink all>> tiddler titles
rp-output: the output of this filter run replaces the output of previous runs unless it is an empty list (see below)
rp-purpose: replace any input to this filter run with its output, only evaluating the run when there is any input
search: 
tags: [[Filter Run Prefix]] [[Filter Syntax]]
title: Then Filter Run Prefix
type: text/vnd.tiddlywiki

\define .op-row()
<$macrocall $name=".if"
	cond="""$(op-body)$"""
	then="""<tr><th align="left">$(op-head)$</th><td><<.op-place>>$(op-body)$</td></tr>"""
	else=""/>
\end

<$list filter="[all[current]has[from-version]]" variable="listItem">
	<$macrocall $name=".from-version" version={{!!from-version}}/>
</$list>
<$let op-head="" op-body="" op-name="">
	<table class="doc-table">
		<!-- purpose -->
		<$let op-head="purpose" op-body={{!!rp-purpose}}>
			<<.op-row>>
		</$let>
		<!-- input -->
		<$let op-head="[[input|Filter Expression]]" op-body={{!!rp-input}}>
			<<.op-row>>
		</$let>
		<!-- suffix -->
		<$let op-head="suffix" op-body={{!!rp-suffix}} op-name={{!!rp-suffix-name}}>
			<<.op-row>>
		</$let>
		<!-- output -->
		<$let op-head="output" op-body={{!!rp-output}}>
			<<.op-row>>
		</$let>
	</table>
</$let>

<$railroad text="""
\start none
\end none
":then"
[[run|"Filter Run"]]
"""/>

!Introduction

The <<.op :then>> filter run prefix is used to replace the result of all previous filter runs with its output.

If the result of all previous runs is an empty list, the <<.op :then>> prefixed filter run is not evaluated.

If the output of a <<.op :then>> prefixed filter run is itself an empty list, the result of all previous filter runs is passed through unaltered.

<<.tip "Note that a list with a single empty string item is not an empty list.">>


!! <<.op :then>> run prefix versus the <<.olink then>> operator

The major difference between the <<.op then>> operator and a <<.op :then>> prefixed filter run is that the operator will replace //each item// of the input [[Title List]] with its parameter while <<.op :then>> will replace the //whole input list// with the result of its run.

|doc-op-comparison tc-center|k
| !<<.op :then>> Filter Run Prefix | !<<.op then>> Operator |
|^<<.operator-example m1-1 "[tag[WikiText]] :then[[true]]">>|^<<.operator-example m1-2 "[tag[WikiText]then[true]]">><p>To make them equivalent, additional filter steps may be added:</p> <<.operator-example m1-3 "[tag[WikiText]count[]compare:number:gt[0]then[true]]">>|


! [[Examples|Then Filter Run Prefix (Examples)]]