Background and Overview

Macros work on the basis of code fragments. The macro system does not understand code fragments; it just substitutes some fragments for other fragments. Once the macro system has substituted and arranged all the code fragments, they are compiled into executable code.

Because the macro system parses and generates code fragments, it can recognize the difference between a string containing a macro name and an actual invocation of a macro. Macros are not affected by such syntactical issues; strings and other expressions are treated as opaque units.

The following are all examples of elementary code fragments, or parsed fragments. These combine to form the larger code fragments upon which macros operate.

'a'
"end times"
35.552
3/4
(3 + 7)
#t
#"red"
red:
cinnamon
==
#[1, 2, 3]
as(...)
list.size

Anatomy and terms

Macros have main rules and auxiliary rules. Each of the main or auxiliary rules has a pattern and a template. Patterns are matched against the code fragments of your source code. A main rule is matched against the code fragment that comprises the entire macro call. An auxiliary rule is matched against parts of that code fragment. The matched code is then replaced by the template.

A pattern can contain code fragments and pattern variables. If a code fragment in the source code matches what is in the pattern, parts of that code fragment may be pulled out into pattern variables. The rest is discarded.

A template can contain other code fragments and substitutions. Substitutions are placeholders; the contents of a pattern variable are processed and inserted into the template in place of every corresponding substitution. The template’s combined fragments and substitutions form the macro’s expansion, which replaces the original code fragment.

This happens recursively: after a macro is expanded, its expansion is scanned for additional macro call code fragments, and those are expanded in turn. The parser recognizes a macro call code fragment by way of a distinguishing word and the type of syntax associated with the macro (discussed further in Macro Types).

Let us examine Function Macro. This macro might be called by the code Original Code, but the actual call fragment Call Fragment is what the parser will attempt to match. The macro’s expansion will be Expansion and the original code will then become Replacement Code.

Here are the parts of the macro:

  • The distinguishing word is table. Whenever the compiler sees table(…), it will expand this macro rather than creating a call to a function named “table”.
  • The main rules are in lines 2–4.
  • The macro has one set of auxiliary rules in lines 6–9. A set of auxiliary rules has a title written as a symbol. This set of auxiliary rules is titled table-contents: (or, alternatively, #"table-contents").
  • The pattern of the first main rule is in line 2.
  • The template of the first main rule is in line 3.
  • The patterns in this macro include the pattern variables ?table-class, ?table-contents, ?rest, ?key, and ?value.
  • The substitutions in this macro include those same names.

Note that the expansion is surrounded by begin and end. Macro expansions are always surrounded by a begin…end block. This helps with macro hygiene (i.e. preventing bindings outside of a macro call from being affected by bindings used in a macro’s expansion). I discuss hygiene here.


Function Macro:

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define macro table
  { table(?table-class:expression, ?table-contents) }
    => { let ht = make(?table-class); ?table-contents; ht; }
  { table(?rest:*) } => { table(<table>, ?rest); }

  table-contents:
  { } => { }
  { ?key:expression => ?value:expression, ... }
    => { ht[?key] := ?value; ... }
end macro table

Original Code:

let lights = table(<string-table>, "red" => "stop", "green" => "go");

Call Fragment:

table(<string-table>, "red" => "stop", "green" => "go")

Expansion:

let ht = make(<string-table>); ht["red"] := "stop"; ht["green"] := "go"; ht;

Replacement Code:

let lights = begin
  let ht = make(<string-table>);
  ht["red"] := "stop"; ht["green"] := "go";
  ht;
end;