4 ms·

Keyword syntax in Scheme dialects

2 points by schemescape 3y ago

schemescape 3y ago
I found it surprising how incompatible the implementations are for Scheme. Are keywords not used as much in Scheme as they are in, say, Common Lisp?
- lispm 3y ago
  Plain Scheme has no special keyword mechanism, unlike Common Lisp. It also has no package-like namespace for symbols and thus no keyword package.
  - schemescape 3y ago
    Thanks!
kazinator 3y ago
In TXR Lisp, the : symbol (keyword with empty string name) plays a big role. It's used as a third value in situations when t and nil aren't enough. In the syntax, it sets off optional lambda parameters: (lambda (a b : c d . r)): two required, two optional and a rest. When an optional parameter is missing an argument, it receives the : value to indicate that, and that triggers the substitution of the default value. The default default value is nil. The : argument value can be explicitly specified. Thus if a function f has three parameters, which are all optional, we can say: (f : : 2) This says, "default the first and second optionals, passing 2 to the third one". A wrapper function can pull the following trick, where the optional parameters specify : itself as the default value: (defun wrapper (a b : (c :) (d :)) (target a b c d)) ;; target also has two required, two optional When you call wrapper as (wrapper 1 2) then the missing c and d default to : which is passed to target. Suppose that looks like: (defun target (a b : (c 3) (d 4)) ...) Then in target, that : value triggers defaulting: so c takes on 3 and d takes on 4. This is exactly what would happen if we called (target 1 2), leaving c and d missing. By using : as the default value for the optionals, the wrapper can propagate their missing status, allowing the target to do all the defaulting. In the tree-bind destructuring binding construct, if a case yields the : value, it means "fall through to the next case", which allows tree-case to do quite a bit more. The printed representation of : (the one the machine produces) is just : whereas in some Common Lisps you see something like :||. The empty name is escaped. I think ANSI CL doesn't allow the pppp:xxxx token syntax with xxxx missing. (2.3.5 Valid Patterns for Tokens). That's shame, since it's such a handy critter.

2 points by schemescape 3y ago

I found it surprising how incompatible the implementations are for Scheme. Are keywords not used as much in Scheme as they are in, say, Common Lisp?

lispm 3y ago
Plain Scheme has no special keyword mechanism, unlike Common Lisp. It also has no package-like namespace for symbols and thus no keyword package.
- schemescape 3y ago
  Thanks!

kazinator 3y ago

In TXR Lisp, the : symbol (keyword with empty string name) plays a big role. It's used as a third value in situations when t and nil aren't enough. In the syntax, it sets off optional lambda parameters: (lambda (a b : c d . r)): two required, two optional and a rest. When an optional parameter is missing an argument, it receives the : value to indicate that, and that triggers the substitution of the default value. The default default value is nil. The : argument value can be explicitly specified. Thus if a function f has three parameters, which are all optional, we can say: (f : : 2) This says, "default the first and second optionals, passing 2 to the third one". A wrapper function can pull the following trick, where the optional parameters specify : itself as the default value: (defun wrapper (a b : (c :) (d :)) (target a b c d)) ;; target also has two required, two optional When you call wrapper as (wrapper 1 2) then the missing c and d default to : which is passed to target. Suppose that looks like: (defun target (a b : (c 3) (d 4)) ...) Then in target, that : value triggers defaulting: so c takes on 3 and d takes on 4. This is exactly what would happen if we called (target 1 2), leaving c and d missing. By using : as the default value for the optionals, the wrapper can propagate their missing status, allowing the target to do all the defaulting. In the tree-bind destructuring binding construct, if a case yields the : value, it means "fall through to the next case", which allows tree-case to do quite a bit more. The printed representation of : (the one the machine produces) is just : whereas in some Common Lisps you see something like :||. The empty name is escaped. I think ANSI CL doesn't allow the pppp:xxxx token syntax with xxxx missing. (2.3.5 Valid Patterns for Tokens). That's shame, since it's such a handy critter.