Template Evaluation

If the evaluator determines that the element is functional position in a form is a template (technically, an expander) then the arguments to the template are not evaluated but passed verbatim to the expander.

The job of the expander/template is to return some expression that can, in turn, be evaluated. (Hence the idea of generating code.) Note that the expression returned must be something that can be evaluated and so cannot be a regular value.

Of course, returned expression might be the invocation of a template in which case its expander will be invoked.

Circular loops should be avoided.

The template is a regular function and can do whatever it likes. The only constraint is that it must return something that can be evaluated. You can create an evaluable return value by hand but it is more convenient to express the evaluable expression as the code you want to have produced: it should look like this but with these elements replaced, ie. a template.

Quasiquoting

quasiquoting is like quote but allows for sections of the quoted expression to be evaluated, like the substitution of a variable name, although any expression can be evaluated.

Quasiquoting can be nested which adds visual complexity.

Idio goes a step further and allows you to change the quasiquotation sigils within a block if it is more convenient.

A quasiquotation block in Idio is delimited by #T{ ... }, denoting a template, and the default quasiquotation sigils are:

• $ for unquoting, that is, have the next expression evaluated, the exact opposite of quote

• @, following the unquote sigil, meaning unquote splicing where the expectation is that the expression being evaluated will return a list and we want all the elements of the list inserted (and not a list of the elements)

• ' for quoting, ie. like quote

• \ for escaping the behaviour of a sigil

You can change the sigils by passing the preferred sigils between #T and the opening {. The default sigils are effectively:

#T$@'\{ ... }

If you only want to change the quoting sigil, say, use . for the preceding two – meaning that . cannot be a sigil:

#T..!{ ... }

Here, the quoting sigil is now ! and the others remain the same.

The expression inside the template is reproduced verbatim except any unquoted elements are evaluated.

Hygiene

Casual creation of variables in templates are at high risk of clashing with or occluding variables outside of the template.

Consider:

define-template (foo a) {
  #T{
    b := frob $a

    printf "b = %s\n" b
  }
}

which will expand into:

b := frob a

printf "b = %s\n" b

where a is whatever was passed to foo when it was invoked.

Here, if b already existed in the environment then we’ve just clobbered it.

The use of gensym is strongly advised. Create a gensymed variable name as a regular variable in the expander function and have the quasiquotation expand the name out to the value:

define-template (foo a) {
  b-var := gensym 'b

  #T{
    $b-var := frob $a

    printf "b = %s\n" $b-var
  }
}

which will now expand into something like:

b/n := frob a

printf "b = %s\n" b/n

where b/n is a unique symbol.

Last built at 2024-12-21T07:10:43Z+0000 from 62cca4c (dev) for Idio 0.3.b.6