1.5.3. Instructions Syntax

In order to produce the instruction we need to define a syntax of how the instuction set should look. This is essentially a cannonical model that describes how words are put together using information from the route description. We will define the syntax using a version of the Backus-Naur-Form (BNF). This is a simple notation of encoding how sentences are structured.

Backus-Naur-Form - Elements

The main element in BNF is the symbol. Symbols are defined using the syntax <SYMBOL_NAME>, where SYMBOL_NAME is the name of a symbol you wish to describe. For example <Start_Instruction>.

A symbol can be defined using other symbols. Here, the definition is made using the notation ::= . Hence, an expression in BNF can be thought of as an equation that relates one symbol to a set of others.

We will use two other types of element in the definitions. Strings are literal phrases that should be included. They will be marked up using double quotes, e.g. "Start on". Objects will relate to the geographic features encoded in the route description. There will be three types of object - landmark, road and junction. They will be marked simply with the word describing the type of object. To indicate a property of an object we will use a . for example road.name indicates the name of a road.

Thus we might define a road link instruction as:

<Road_Link_Instruction> ::= "Continue along" road.name "for" road.length "passing" landmark.name

which might then produce a text instruction such as:
"Continue along Werdmuehlestrasse for 200 metres passing Spielzeug Museum".

Backus-Naur-Form - Operators

There are a number of useful operators that we will also need to use. The most important is the "or" sign | . This means that the instruction should use one of a number of options. () Brackets can be used to indicate a list of options.

For example we could define a symbol:

<Turn_Indicator> ::= "turn" ("left" | "right") "onto" road.name

which might be used to produce:
"turn left onto Werdmuehlestrasse".

Another operator is known as the Kleene star. The notation is ( )^* This indicates that a symbol or set of symbols should be repeated. For example:

<Road_Link_Instruction> ::= "Continue along" road.name "for" road.length "passing" (landmark.name "on your" ("left"|"right"))^*

might be used to produce an instruction such as:
"Continue along Werdmuehlestrasse for 200 metres passing Spielzeug Museum on your left, Museum Strauhof on your right".

Finally, we will use the optional operator which is indicated using square brackets [ ]. This means that the definition in the brackets is optional for the sentence. For example we might place the the landmark part of the previous symbol in brackets since there may be no landmarks found along a particular stretch of road, i.e.:

<Road_Link_Instruction> ::= "Continue along" road.name "for" road.length ["passing" (landmark.name "on your" ("left"|"right"))^*]

BNF Summary