Commands

A command in StackLang is stored as a string. Commands can be either quoted or unquoted. Unquoted commands on the stack are automatically evaluated, while quoted commands on the stack are treated as data. It is not possible to have an unquoted command at top level in the stack without it being evaluated, outside of error cases.

An EBNF definition of commands is given below.

alphabetic-character = ? Any ASCII character in the ranges (inclusive) a-z, A-Z ? ;
numeric-character = ? Any ASCII character in the ranges (inclusive) 0-9 ? ;
symbolic-character = "-" | "?" | "*" ;
character = alphabetic-character | numeric-character | symbolic-character ;

quoted-symbol = "`" ;

quoted-command = quoted-symbol, alphabetic-character, {character} ;
unquoted-command = alphabetic-character, {character} ;

command = quoted-command | unquoted-command ;

Command-related Commands

Type Predicates

command? : Any -> Boolean
Produces true if input is a command. In practice, this is equivalent to quoted?.

quoted? : Any -> Boolean
Produces true if input is a quoted command.

defined?: Command -> Boolean
Produces true if given command is defined.

local? : Any -> Boolean
Produces true if define bound to this command has a context. Command must exist - fails with a RuntimeError if command does not exist.

Type Conversion

unquote : Command -> Command
Turns a quoted command into a command, which, by the evaluation rules, is immediately evaluated. No command exists that quotes a command, since unquoted commands can't exist on the stack.

command-to-string : Command -> String
Produces the string-form representation of the command. Includes the inital grave symbol, `.

string-to-command : String -> Command
Parses a command, as if it were read in from the interpreter. Fails with a RuntimeError if string cannot be parsed. If resulting command is not quoted, result is immediately evaluated.

string-to-command* : String -> Command
Parses a command, as if it were read in from the interpreter. Fails with a RuntimeError if string cannot be parsed. Resulting command is coerced into quoted form automatically.

Command Properties

For all of these commands, the given command must exist, and these commands will fail with a RuntimeError if the command does not exist.

arity : Command -> Number
Produces the arity of the given command.

body : Command -> Substack
Produces the body of this command. Fails with a RuntimmeError if given command is not defined.

context : Command -> Command
Produces the context of this command. Fails with a RuntimeError if none exists.

signature : Command -> Substack(Types)
Produces the signature of this command (the list of types it expects, in order).

Utilities

gensym : -> Command

lambda : Substack Substack(Type) -> Command
Defines a function in the global context with a name created by gensym, and produces the name.

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