Macros

Passerine has a rich hygienic* syntactic macro system that extends the language itself.

Syntactic macros, quite simply, are bits of code that hygienically produce more code when invoked at compile time. Macros use a small, simple-yet-powerful set of rules to transform code.

* Having read Doug Hoyte's exellent Let Over Lambda, I understand the raw power of a rich unhygenic macro system. However, such systems are hard to comprehend, and harder to master. Passerine aims to be as simple and powerful as possible without losing transparency: hygienic macro systems are much more transparent then their opaque unhygenic counterparts.

Extensions are defined with the syntax keyword, followed by some argument patterns, followed by the code that the captured arguments will be spliced into. Here's a simple example: we're using a macro to define swap operator:

syntax a 'swap b {
    tmp = a
    a = b
    b = tmp
}

x = 7
y = 3
x swap y

Note that the above code is completely hygienic. the expanded macro looks something like this:

_tmp = x
x = y
x = _tmp

Because tmp was not passed as a macro pattern parameter, all uses of tmp in the macro body are unique unrepresentable variables that do not collide with any other variables currently bound in scope. Essentially:

tmp = 1
x = 2
y = 3
x swap y

Will not affect the value of tmp; tmp will still be 1.

Argument Patterns

So, what is an argument pattern (an arg-pat)? Arg-pats are what go between:

syntax ... { }

Each item between syntax and the macro body is an arg-pat. Arg-pats can be:

Syntactic variables, like foo and bar.
Literal syntactic identifiers, which are prefixed with a quote ('), like 'let.
Nested argument patterns, followed by optional modifiers.

Let's start with syntactic identifiers. Identifiers are literal names that must be present for the pattern to match. Each syntactic extension is required to have at least one. For example, here's a macro that matches a for loop:

syntax 'for binding 'in values do { ... }

In this case, 'for and 'in are syntactic identifiers. This definition could be used as follows:

for a in [1, 2, 3] {
    print a
}

Syntactic variables are the other identifiers in the pattern that are bound to actual values. In the above example, a → binding, [1, 2, 3] → values, and { print a } → do.

Macros can also be used to define operators†:

syntax sequence 'contains value {
    c = seq -> match seq {
        [] -> False
        [head, ..] | head == value -> True
        [_, ..tail] -> c tail
    }

    c sequence
}

This defines a contains operator that could be used as follows:

print {
    if [1, 2, 3] contains 2 {
        "It contains 2"
    } else {
        "It does not contain 2"
    }
}

Evidently, It contains 2 would be printed.

† Custom operators defined in this manner will always have the lowest precedence, and must be explicitly grouped when ambiguous. For this reason, Passerine already has a number of built-in operators (with proper precedence) which can be overloaded. It's important to note that macros serve to introduce new constructs that just happen to be composable – syntactic macros can be used to make custom operators, but they can be used for so much more. I think this is a fair trade-off to make.

Modifiers are postfix symbols that allow for flexibility within argument patterns. Here are some modifiers:

Zero or more (...)
Optional (?)

Additionally, parenthesis are used for grouping, and { ... } are used to match expressions within blocks. Let's construct some syntactic arguments that match an if else statement, like this one:

if x == 0 {
    print "Zero"
} else if x % 2 == 0 {
    print "Even"
} else {
    print "Not even or zero"
}

The arg-pat must match a beginning if:

syntax 'if { ... }

Then, a condition:

syntax 'if condition { ... }

Then, the first block:

syntax 'if condition then { ... }

Next, we'll need a number of else if <condition> statements:

syntax 'if condition then ('else 'if others do)... { ... }

Followed by a required closing else (Passerine is expression oriented and type-checked, so a closing else ensures that an if expression always returns a value.):

syntax 'if condition then ('else 'if others do)... 'else finally { ... }

Of course, if statements are already baked into the language – let's build something else – a match expression.

The Passerine Codex

Macros

Hygiene

Argument Patterns