created: 20150103184022184
modified: 20150119220342000
title: $:/plugins/tiddlywiki/railroad/syntax
type: text/vnd.tiddlywiki

The railroad widget uses a special notation to construct the components defined below.

`x` and `y` here stand for any component.

Names (as opposed to quoted strings) are available when a value starts with a letter and contains only letters, digits, underscores, dots and hyphens.

---

; sequence
: <$railroad text=""" ["<-"] {x} ["->"] """/>
* A sequence of components
* The `<-` and `->` delimiters allow you to force a single component to be treated as a sequence. This is occasionally useful for spacing a diagram out

---

; optional
: <$railroad text=""" "[" [":"] x "]" """/>
* A component that can be omitted
* The colon makes `x` appear straight ahead

---

; repeated
: <$railroad text=""" "{" x [:"+" y] "}" """/>
* A list of one or more `x`
* The `+` suffix adds `y` as a separator between each `x` and the next

---

; optional repeated
: <$railroad text=""" "[{" [":"] x [:"+" y] "}]" """/>
* An optional list of `x`, i.e. a list of zero or more `x`

---

; choice
: <$railroad text=""" "(" {[:":"] x +"|"} ")" """/>
* A set of alternatives
* The colon indicates which branch appears straight ahead. By default, it's the first branch

---

; string / terminal
: <$railroad text={{$:/plugins/tiddlywiki/railroad/syntax-string}}/>
* A literal or terminal component
* This follows the normal ~TiddlyWiki rules for quoted strings

---

; nonterminal
: <$railroad text=""" (name | "<" string ">") """/>
* A nonterminal component, i.e. the name of another diagram

---

; comment
: <$railroad text=""" "/" string "/" """/>
* A comment

---

; dummy
: <$railroad text=""" "-" """/>
* The absence of a component

---

; link
: <$railroad text=""" "[[" x "|" (name|string) "]]" """/>
* A link to the tiddler title or URI given by the string or name

---

; transclusion
: <$railroad text=""" "{{" (name|string) "}}" """/>
* Treats the content of another tiddler as diagram syntax and transcludes it into the current diagram

---

; arrow pragma
: <$railroad text=""" "\arrow" ("yes" | "no") """/>
* Controls whether repeat paths have an arrow on them
* Can be toggled on and off in mid-diagram, if desired

---

; debug pragma
: <$railroad text=""" "\debug" """/>
* Causes the diagram to display its parse tree

---

; start/end pragma
: <$railroad text=""" ("\start" |: "\end") ("none" |: "single" | "double") """/>
* Controls the style of the diagram's startpoint or endpoint