4.8 Macros

4.8 Macros

The third category of construct appearing at the highest syntactic level in a

FunnelWeb input file is the macro definition. A macro definition binds a

Tutorial

unique 

macro name

 to a 

macro body

 containing an 

expression

consisting of text, calls to other macros, and formal parameters. The

Developer

syntax for a macro definition is as follows:

Reference

macro = ("@O" | "@$") name [formal_parameter_list]

1 Introduction

        ["@Z"] ["@M"] { "@L" } ["==" | "+="]

2 Interface

        "@{" expression "@}"

3 Scanner

The complexity of the macro definition syntax is mostly to enable the user

4 Parser

to attach various attributes to the macro. If the user chooses @O, then the

5 Analyser

macro cannot be called, but is instead attached to a product file. If the user

6 Tangle

chooses @$, then the macro is an ordinary macro definition that is not

7 Weave

attached to a file. Here are some example macro definitions.

8 Shell

9 Commands

@O@@{This is an @.@}

10 Glossary

@$@@M@{swan@}

11 References

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Number Of Invocations Constraint

By default, a non file macro must be invoked exactly once by one other

macro. Macros that aren t are flagged with errors by the FunnelWeb

analyser. However, if the user uses the @Z sequence in the macro

definition, the macro is then permitted to be invoked zero times, as well as

once. Similarly, if the user uses the @M sequence in the macro definition,

the macro is permitted to be called many times as well as once. If both @Z

and @M are present then the macro is permitted to be invoked zero, one,

or many times.

The purpose of enforcing the default "exactly one call" rule is to flag

pieces of code that the user may have defined in a macro but not hooked

into the rest of the program. Experience shows that this is a common error.

Similarly, it can be dangerous to multiply invoke a macro intended to be

invoked only once. For example, it may be dangerous to invoke a scrap of

non idempotent initialization code in two different parts of the main

function of a program! However, FunnelWeb will not generate an error if

a macro without @M is called by another macro that is called more than

once.

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