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| -rw-r--r-- | LANGUAGE.rst | 57 |
1 files changed, 47 insertions, 10 deletions
diff --git a/LANGUAGE.rst b/LANGUAGE.rst index ed3e942..1ed3400 100644 --- a/LANGUAGE.rst +++ b/LANGUAGE.rst @@ -169,8 +169,8 @@ Lists that begin with one of the following symbols are evaluated specially. decimal, a non-empty string, or a non-nil ``cons``), then the ``if`` form evaluates the ``true`` subform. - * If the ``cond`` subform produces a false value (any other value), then - the ``if`` form evaluates the ``false`` subform. + * If the ``cond`` subform evaluates to a false value (any other value), + then the ``if`` form evaluates the ``false`` subform. * If the ``if`` form does not have a ``false`` subform, the ``if`` form evaluates to ``nil`` when the ``cond`` subform evaluates to a false @@ -185,8 +185,8 @@ Lists that begin with one of the following symbols are evaluated specially. * Must include a ``formals`` subform, which is generally a list of argument names (as symbols). - * May include a sequence of body subforms, which are evaluated in order - whenever the function is applied. + * May include a sequence of body subforms, which are evaluated in order (as + if by ``begin``) whenever the function is applied. * Functions capture the environment in effect when they are defined. Symbols within the function body can refer to names defined in the @@ -223,7 +223,8 @@ Lists that begin with one of the following symbols are evaluated specially. for the ``+`` function itself, but it illustrates the idea that functions can include other functions. -* ``define``: declares new bindings. This has two formats: +* ``define``: A ``declare`` form sets the value of a new binding in the current + environment. This has two forms: * ``(define symbol value)``: evaluates the ``value`` subform, and binds the result to ``symbol`` in the current environment. Example: @@ -365,11 +366,47 @@ Macros (let-one (x 1) x) + The macro procedure accepts the forms ``(x 1)`` and ``x``, unevaluated, as + arguments, and substitutes them into a *quasiquoted* form, which is used as a + template. The three *unquoted* parts (``,name``, ``,body``, and ``val``) are + replaced by evaluating the symbols in the context of the macro procedure, and + expand to the relevant parts of the input forms. + + The returned form is approximately + +:: + + ((lambda (x) x) 1) + + and evaluates as such. + This program evaluates to 1, but *does not* bind ``x`` in the top-level environment. -TO WRITE: - -* Comprehensive intro to quote forms -* Defining a simple macro -* Applying a simple macro +* Actinide macros are *not hygienic*. A quoted symbol in the macro body will be + evaluated in the location where the macro is expanded, with full access to + the environment at that location. Similarly, symbols defined in the macro + will be fully visible to code running in the environment where the macro is + expanded. + +* Macros often use quote notation to build the returned form. Quote notation is + ultimately a sequence of ``quote`` forms. However, Actinide supports + *quasiquote* notation to simplify the creation of nested quoted forms + containing unquoted parts. + + * A quasiquote form begins with ``\```. If the form contains no unquoted + parts, this will quasiquote each subform, terminating by quoting each + symbol or literal form and ``cons``ing them into a new list. ``\`(a b)`` + expands to ``('a 'b)``.` + + * Within a quasiquote form, an *unquote* form prevents the following form + from being quoted. An unquote form begins with ``,``, followed by a + single form (often, but not always, a single symbol). ``\`(a ,b c)`` + expands to ``('a b 'c)``. + + * Within a quasiquote form, an *unquote-splicing* form prevents the + following form from being quoted. An unquote-splicing form begins with + ``,@``, followed by a single form, which must evaluate to a list. The + elements of that list are grafted into the resulting form. Given + ``(define x (list 1 2))``, the form ``\`(a ,@x b)`` expands to ``('a 1 2 + 'b)``. |