In the "Dictio" file, located at the link "Text-only console version" of this site, I've noticed a Lisp command (?) called defs.
I assume that this is something similar to defun, but am unable to find any information on what defs does; is it used to define a function, or maybe a variable? I am looking to reproduce this code using modern techniques and it would help to know the purpose of defs.
The defs calls seem to also include more than a name before the arguments (I would expect it to read (defs name () body).
Looking at the first function in the list, there appears to be more included in this "function definition" [the word 'features' specifically] and in the third function, there is ['semantics'] included after what appears to be the name of the function (before the arguments).
DEFS is defined by the software in the file SYSCOM.
It is a FEXPR, which is a function which gets the arguments unevaluated. Common Lisp has no such feature. It uses macros instead.
Example use:
(DEFS \#COLOR
FEXPR (LAMBDA (A)
(EVAL (SUBST (CAR A)
'COLOR
'(OBJECT
(MARKERS\: (\#PHYSOB COLOR)
PROCEDURE\: ((\#COLOR *** COLOR)))))))
PRIORITY 192.
SYS (\#PROPERTY))
Here you have a symbol #COLOR. It gets a function (actually a FEXPR) defined under this name. Also it puts a PRIORITY and SYS onto the property list of the symbol. Thus DEFS is used to define symbols with functions and properties in one defining form.
Related
I'm learning blocks in Common lisp and did this example to see how blocks and the return-from command work:
(block b1
(print 1)
(print 2)
(print 3)
(block b2
(print 4)
(print 5)
(return-from b1)
(print 6)
)
(print 7))
It will print 1, 2, 3, 4, and 5, as expected. Changing the return-from to (return-from b2) it'll print 1, 2, 3, 4, 5, and 7, as one would expect.
Then I tried turn this into a function and paremetrize the label on the return-from:
(defun test-block (arg) (block b1
(print 1)
(print 2)
(print 3)
(block b2
(print 4)
(print 5)
(return-from (eval arg))
(print 6)
)
(print 7)))
and using (test-block 'b1) to see if it works, but it doesn't. Is there a way to do this without conditionals?
Using a conditional like CASE to select a block to return from
The recommended way to do it is using case or similar. Common Lisp does not support computed returns from blocks. It also does not support computed gos.
Using a case conditional expression:
(defun test-block (arg)
(block b1
(print 1)
(print 2)
(print 3)
(block b2
(print 4)
(print 5)
(case arg
(b1 (return-from b1))
(b2 (return-from b2)))
(print 6))
(print 7)))
One can't compute lexical go tags, return blocks or local functions from names
CLTL2 says about the restriction for the go construct:
Compatibility note: The ``computed go'' feature of MacLisp is not supported. The syntax of a computed go is idiosyncratic, and the feature is not supported by Lisp Machine Lisp, NIL (New Implementation of Lisp), or Interlisp. The computed go has been infrequently used in MacLisp anyway and is easily simulated with no loss of efficiency by using a case statement each of whose clauses performs a (non-computed) go.
Since features like go and return-from are lexically scoped constructs, computing the targets is not supported. Common Lisp has no way to access lexical environments at runtime and query those. This is for example also not supported for local functions. One can't take a name and ask for a function object with that name in some lexical environment.
Dynamic alternative: CATCH and THROW
The typically less efficient and dynamically scoped alternative is catch and throw. There the tags are computed.
I think these sorts of things boils down to the different types of namespaces bindings and environments in Common Lisp.
One first point is that a slightly more experienced novice learning Lisp might try to modify your attempted function to say (eval (list 'return-from ,arg)) instead. This seems to make more sense but still does not work.
Namespaces
A common beginner mistake in a language like scheme is having a variable called list as this shadows the top level definition of this as a function and stops the programmer from being able to make lists inside the scope for this binding. The corresponding mistake in Common Lisp is trying to use a symbol as a function when it is only bound as a variable.
In Common Lisp there are namespaces which are mappings from names to things. Some namespaces are:
The functions. To get the corresponding thing either call it: (foo a b c ...), or get the function for a static symbol (function foo) (aka #'foo) or for a dynamic symbol (fdefinition 'foo). Function names are either symbols or lists of setf and one symbol (e.g. (serf bar)). Symbols may alternatively be bound to macros in this namespace in which case function and fdefinition signal errors.
The variables. This maps symbols to the values in the corresponding variable. This also maps symbols to constants. Get the value of a variable by writing it down, foo or dynamically as (symbol-value). A symbol may also be bound as a symbol-macro in which case special macro expansion rules apply.
Go tags. This maps symbols to labels to which one can go (like goto in other languages).
Blocks. This maps symbols to places you can return from.
Catch tags. This maps objects to the places which catch them. When you throw to an object, the implementation effectively looks up the corresponding catch in this namespace and unwinds the stack to it.
classes (and structs, conditions). Every class has a name which is a symbol (so different packages may have a point class)
packages. Each package is named by a string and possibly some nicknames. This string is normally the name of a symbol and therefore usually in uppercase
types. Every type has a name which is a symbol. Naturally a class definition also defines a type.
declarations. Introduced with declare, declaim, proclaim
there might be more. These are all the ones I can think of.
The catch-tag and declarations namespaces aren’t like the others as they don’t really map symbols to things but they do have bindings and environments in the ways described below (note that I have used declarations to refer to the things that have been declared, like the optimisation policy or which variables are special, rather than the namespace in which e.g. optimize, special, and indeed declaration live which seems too small to include).
Now let’s talk about the different ways that this mapping may happen.
The binding of a name to a thing in a namespace is the way in which they are associated, in particular, how it may come to be and how it may be inspected.
The environment of a binding is the place where the binding lives. It says how long the binding lives for and where it may be accessed from. Environments are searched for to find the thing associated with some name in some namespace.
static and dynamic bindings
We say a binding is static if the name that is bound is fixed in the source code and a binding is dynamic if the name can be determined at run time. For example let, block and tags in a tagbody all introduce static bindings whereas catch and progv introduce dynamic bindings.
Note that my definition for dynamic binding is different from the one in the spec. The spec definition corresponds to my dynamic environment below.
Top level environment
This is the environment where names are searched for last and it is where toplevel definitions go to, for example defvar, defun, defclass operate at this level. This is where names are looked up last after all other applicable environments have been searched, e.g. if a function or variable binding can not be found at a closer level then this level is searched. References can sometimes be made to bindings at this level before they are defined, although they may signal warnings. That is, you may define a function bar which calls foo before you have defined foo. In other cases references are not allowed, for example you can’t try to intern or read a symbol foo::bar before the package FOO has been defined. Many namespaces only allow bindings in the top level environment. These are
constants (within the variables namespace)
classes
packages
types
Although (excepting proclaim) all bindings are static, they can effectively be made dynamic by calling eval which evaluates forms at the top level.
Functions (and [compiler] macros) and special variables (and symbol macros) may also be defined top level. Declarations can be defined toplevel either statically with the macro declaim or dynamically with the function proclaim.
Dynamic environment
A dynamic environment exists for a region of time during the programs execution. In particular, a dynamic environment begins when control flow enters some (specific type of) form and ends when control flow leaves it, either by returning normally or by some nonlocal transfer of control like a return-from or go. To look up a dynamically bound name in a namespace, the currently active dynamic environments are searched (effectively, ie a real system wouldn’t be implemented this way) from most recent to oldest for that name and the first binding wins.
Special variables and catch tags are bound in dynamic environments. Catch tags are bound dynamically using catch while special variables are bound statically using let and dynamically using progv. As we shall discuss later, let can make two different kinds of binding and it knows to treat a symbol as special if it has been defined with defvar or ‘defparameteror if it has been declared asspecial`.
Lexical environment
A lexical environment corresponds to a region of source code as it is written and a specific runtime instantiation of it. It (slightly loosely) begins at an opening parenthesis and ends at the corresponding closing parenthesis, and is instantiated when control flow hits the opening parenthesis. This description is a little complicated so let’s have an example with variables which are bound in a lexically environment (unless they are special. By convention the names special variables are wrapped in * symbols)
(defun foo ()
(let ((x 10))
(bar (lambda () x))))
(defun bar (f)
(let ((x 20))
(funcall f)))
Now what happens when we call (foo)? Well if x were bound in a dynamic environment (in foo and bar) then the anonymous function would be called in bar and the first dynamic environment with a binding for x would have it bound to 20.
But this call returns 10 because x is bound in a lexical environment so even though the anonymous function gets passed to bar, it remembers the lexical environment corresponding to the application of foo which created it and in that lexical environment, x is bound to 10. Let’s now have another example to show what I mean by ‘specific runtime instantiation’ above.
(defun baz (islast)
(let ((x (if islast 10 20)))
(let ((lx (lambda () x)))
(if islast
lx
(frob lx (baz t))))))
(defun frob (a b)
(list (funcall a) (funcall b)))
Now running (baz nil) will give us (20 10) because the first function passed to frob remembers the lexical environment for the outer call to baz (where islast is nil) whilst the second remembers the environment for the inner call.
For variables which are not special, let creates static lexical bindings. Block names (introduced statically by block), go tags (scopes inside a tagbody), functions (by felt or labels), macros (macrolet), and symbol macros (symbol-macrolet) are all bound statically in lexical environments. Bindings from a lambda list are also lexically bound. Declarations can be created lexically using (declare ...) in one of the allowed places or by using (locally (declare ...) ...) anywhere.
We note that all lexical bindings are static. The eval trick described above does not work because eval happens in the toplevel environment but references to lexical names happen in the lexical environment. This allows the compiler to optimise references to them to know exactly where they are without running code having to carry around a list of bindings or accessing global state (e.g. lexical variables can live in registers and the stack). It also allows the compiler to work out which bindings can escape or be captured in closures or not and optimise accordingly. The one exception is that the (symbol-)macro bindings can be dynamically inspected in a sense as all macros may take an &environment parameter which should be passed to macroexpand (and other expansion related functions) to allow the macroexpander to search the compile-time lexical environment for the macro definitions.
Another thing to note is that without lambda-expressions, lexical and dynamic environments would behave the same way. But note that if there were only a top level environment then recursion would not work as bindings would not be restored as control flow leaves their scope.
Closure
What happens to a lexical binding captured by an anonymous function when that function escapes the scope it was created in? Well there are two things that can happen
Trying to access the binding results in an error
The anonymous function keeps the lexical environment alive for as long as the functions referencing it are alive and they can read and write it as they please.
The second case is called a closure and happens for functions and variables. The first case happens for control flow related bindings because you can’t return from a form that has already returned. Neither happens for macro bindings as they cannot be accessed at run time.
Nonlocal control flow
In a language like Java, control (that is, program execution) flows from one statement to the next, branching for if and switch statements, looping for others with special statements like break and return for certain kinds of jumping. For functions control flow goes into the function until it eventually comes out again when the function returns. The one nonlocal way to transfer control is by using throw and try/catch where if you execute a throw then the stack is unwound piece by piece until a suitable catch is found.
In C there are is no throw or try/catch but there is goto. The structure of C programs is secretly flat with the nesting just specifying that “blocks” end in the opposite order to the order they start. What I mean by this is that it is perfectly legal to have a while loop in the middle of a switch with cases inside the loop and it is legal to goto the middle of a loop from outside of that loop. There is a way to do nonlocal control transfer in C: you use setjmp to save the current control state somewhere (with the return value indicating whether you have successfully saved the state or just nonlocally returned there) and longjmp to return control flow to a previously saved state. No real cleanup or freeing of memory happens as the stack unwinds and there needn’t be checks that you still have the function which called setjmp on the callstack so the whole thing can be quite dangerous.
In Common Lisp there’s a range of ways to do nonlocal control transfer but the rules are more strict. Lisp doesn’t really have statements but rather everything is built out of a tree of expressions and so the first rule is that you can’t nonlocally transfer control into a deeper expression, you may only transfer out. Let’s look at how these different methods of control transfer work.
block and return-from
You’ve already seen how these work inside a single function but recall that I said block names are lexically scoped. So how does this interact with anonymous functions?
Well suppose you want to search some big nested data structure for something. If you were writing this function in Java or C then you might implement a special search function to recurse through your data structure until it finds the right thing and then return it all the way up. If you were implementing it in Haskell then you would probably want to do it as some kind of fold and rely on lazy evaluation to not do too much work. In Common Lisp you might have a function which applies some other function passed as a parameter to each item in the data structure. And now you can call that with a searching function. How might you get the result out? Well just return-from to the outer block.
tagbody and go
A tagbody is like a progn but instead of evaluating single symbols in the body, they are called tags and any expression within the tagbody can go to them to transfer control to it. This is partly like goto, if you’re still in the same function but if your go expression happens inside some anonymous function then it’s like a safe lexically scoped longjmp.
catch and throw
These are most similar to the Java model. The key difference between block and catch is that block uses lexical scoping and catch uses dynamic scoping. Therefore their relationship is like that between special and regular variables.
Finally
In Java one can execute code to tidy things up if the stack has to unwind through it as an exception is thrown. This is done with try/finally. The Common Lisp equivalent is called unwind-protect which ensures a form is executed however control flow may leave it.
Errors
It’s perhaps worth looking a little at how errors work in Common Lisp. Which of these methods do they use?
Well it turns out that the answer is that errors instead of generally unwinding the stack start by calling functions. First they look up all the possible restarts (ways to deal with an error) and save them somewhere. Next they look up all applicable handlers (a list of handlers could, for example, be stored in a special variable as handlers have dynamic scope) and try each one at a time. A handler is just a function so it might return (ie not want to handle the error) or it might not return. A handler might not return if it invokes a restart. But restarts are just normal functions so why might these not return? Well restarts are created in a dynamic environment below the one where the error was raised and so they can transfer control straight out of the handler and the code that threw the error to some code to try to do something and then carry on. Restarts can transfer control using go or return-from. It is worth noting that it is important here that we have lexical scope. A recursive function could define a restart on each successive call and so it is necessary to have lexical scope for variables and tags/block names so that we can make sure we transfer control to the right level on the call stack with the right state.
defmacro is documented at http://clhs.lisp.se/Body/m_defmac.htm but the documentation is not entirely clear on exactly when things happen. By experiment with Clisp, I have found the following (assuming all macros and functions defined at top level):
Straight top-level code can only call macros and functions that have been defined earlier.
Code within a macro or function, or generated by a macro, can call any function it likes, including one define later (as expected from the need to support mutual recursion).
Code within a macro can only call a macro defined earlier than the calling site of the first macro.
Code generated by a macro can call a macro defined later.
Is it the case that Clisp is just following the specification, or is there any variation between implementations in this regard?
Is the exact intended set of rules, and the rationale behind them, documented anywhere?
You are asking about macro expansion - but I'd like to clarify how functions are handled first.
Pay attention to when the calls and the defines actually happens. In your second point you say code within a function can call a function that is defined later. This isn't strictly true.
In languages like C++ you declare and define functions and then compile your app. Ignoring inlining, templates, lambdas and other magic..., when compiling a function, the declarations of all other functions used by that function need to be present - and at link time, the compiled definitions need to be present - all before the program starts running. Once the program starts running, all functions are already fully prepared and ready to be called.
Now in Lisp, things are different. Ignore compilation for now - let's just think about an interpreted environment. If you run:
;; time 1
(defun a () (b))
;; time 2
(defun b () 123)
;; time 3
(a)
At time 1 your program has no functions.
The first defun then creates a function (lambda () (b)), and associates it with the symbol a. This function contains a reference to the symbol b, but at this point in time it is not calling b. a will only call b when a itself gets called.
So, at time 2 your program has one function, associated with the symbol a, but it has not been executed yet.
Now the second defun creates a function (lambda () 123), and associates it with the symbol b.
At time 3 your program has two functions, associated with the symbols a and b, but neither has been called yet.
Now you call a. During its execution, it looks for the function associated with the symbol b, finds that such a function already exists at this point in time, and calls it. b executes and returns 123.
Let's add more code:
;; time 4
(defun b () 456)
;; time 5
(a)
After time 4, a new defun creates a function returning 456, and associates it with the symbol b. This replaces the reference b was holding to the function returning 123, which will then be garbage collected (or whatever you implementation does to take out the trash).
Calling a (or more correctly, the lambda referenced by the function attribute of the symbol a), will now result in a call to a function that returns 456.
If, instead, we had originally written:
;; time 1
(defun a () (b))
;; time 2
(a)
;; time 3
(defun b () 123)
... this would not have worked, because after time 2 when we call a, it can't find a function associated with the symbol b and so it will fail.
Now - compile, eval-when, optimisation and other magic can do all kinds of funky things different from what I've described above, but make sure you first have a grasp of these basics before worrying about that more advanced stuff.
Functions are only created at the time that defun is called. (The interpreter doesn't "look ahead in the file".)
One of the attributes of a symbol is a reference to a function. (The function itself doesn't actually have a name.)
Multiple symbols can reference the same function. ((setf (symbol-function 'd) (symbol-function 'b)))
Defining a function a that calls function b (speaking colloquially), is OK as long as the symbol b has an associated function by the time a is called. (It is not required at the time of defunning a.)
A symbol can refer to different functions at different times. This affects any functions "calling" that symbol.
The rules for macros are different (their expansions are static after "read" time), but many of the principles remain the same (Lisp doesn't "look ahead in the file" to find them). Understand that Lisp programs are far more dynamic and "run-time" than most (lesser ;-) ) languages you may be used to. Understand what happens when during execution of a Lisp program, and the rules governing macro expansion will start making sense.
The question is not about using keywords, but actually about keyword implementation. For example, when I create some function with keyword parameters and make a call:
(defun fun (&key key-param) (print key-param)) => FUN
(find-symbol "KEY-PARAM" 'keyword) => NIL, NIL ;;keyword is not still registered
(fun :key-param 1) => 1
(find-symbol "KEY-PARAM" 'keyword) => :KEY-PARAM, :EXTERNAL
How a keyword is used to pass an argument? Keywords are symbols whos values are themselves, so how a corresponding parameter can be bound using a keyword?
Another question about keywords — keywords are used to define packages. We can define a package named with already existing keyword:
(defpackage :KEY-PARAM) => #<The KEY-PARAMETER package, 0/16 ...
(in-package :KEY-PARAM) => #<The KEY-PARAMETER package, 0/16 ...
(defun fun (&key key-param) (print key-param)) => FUN
(fun :KEY-PARAM 1) => 1
How does system distinguish the usage of :KEY-PARAM between package name and function parameter name?
Also we can make something more complicated, if we define function KEY-PARAM and export it (actually not function, but name):
(in-package :KEY-PARAM)
(defun KEY-PARAM (&key KEY-PARAM) KEY-PARAM) => KEY-PARAM
(defpackage :KEY-PARAM (:export :KEY-PARAM))
;;exporting function KEY-PARAM, :KEY-PARAM keyword is used for it
(in-package :CL-USER) => #<The COMMON-LISP-USER package, ...
(KEY-PARAM:KEY-PARAM :KEY-PARAM 1) => 1
;;calling a function KEY-PARAM from :KEY-PARAM package with :KEY-PARAM parameter...
The question is the same, how Common Lisp does distinguish the usage of keyword :KEY-PARAM here?
If there is some manual about keywords in Common Lisp with explanation of their mechanics, I would be grateful if you posted a link here, because I could find just some short articles only about usage of the keywords.
See Common Lisp Hyperspec for full details of keyword parameters. Notice that
(defun fun (&key key-param) ...)
is actually short for:
(defun fun (&key ((:key-param key-param)) ) ...)
The full syntax of a keyword parameter is:
((keyword-name var) default-value supplied-p-var)
default-value and supplied-p-var are optional. While it's conventional to use a keyword symbol as the keyword-name, it's not required; if you just specify a var instead of (keyword-name var), it defaults keyword-name to being a symbol in the keyword package with the same name as var.
So, for example, you could do:
(defun fun2 (&key ((myoption var))) (print var))
and then call it as:
(fun 'myoption 3)
The way it works internally is when the function is being called, it steps through the argument list, collecting pairs of arguments <label, value>. For each label, it looks in the parameter list for a parameter with that keyword-name, and binds the corresponding var to value.
The reason we normally use keywords is because the : prefix stands out. And these variables have been made self-evaluating so that we don't have to quote them as well, i.e. you can write :key-param instead of ':key-param (FYI, this latter notation was necessary in earlier Lisp systems, but the CL designers decided it was ugly and redundant). And we don't normally use the ability to specify a keyword with a different name from the variable because it would be confusing. It was done this way for full generality. Also, allowing regular symbols in place of keywords is useful for facilities like CLOS, where argument lists get merged and you want to avoid conflicts -- if you're extending a generic function you can add parameters whose keywords are in your own package and there won't be collisions.
The use of keyword arguments when defining packages and exporting variables is again just a convention. DEFPACKAGE, IN-PACKAGE, EXPORT, etc. only care about the names they're given, not what package it's in. You could write
(defpackage key-param)
and it would usually work just as well. The reason many programmers don't do this is because it interns a symbol in their own package, and this can sometimes cause package conflicts if this happens to have the same name as a symbol they're trying to import from another package. Using keywords divorces these parameters from the application's package, avoiding potential problems like this.
The bottom line is: when you're using a symbol and you only care about its name, not its identity, it's often safest to use a keyword.
Finally, about distinguishing keywords when they're used in different ways. A keyword is just a symbol. If it's used in a place where the function or macro just expects an ordinary parameter, the value of that parameter will be the symbol. If you're calling a function that has &key arguments, that's the only time they're used as labels to associate arguments with parameters.
The good manual is Chapter 21 of PCL.
Answering your questions briefly:
keywords are exported symbols in keyword package, so you can refer to them not only as :a, but also as keyword:a
keywords in function argument lists (called lambda-lists) are, probably, implemented in the following way. In presence of &key modifier the lambda form is expanded into something similar to this:
(let ((key-param (getf args :key-param)))
body)
when you use a keyword to name a package it is actually used as a string-designator. This is a Lisp concept that allows to pass to a certain function dealing with strings, that are going to be used as symbols later (for different names: of packages, classes, functions, etc.) not only strings, but also keywords and symbols. So, the basic way to define/use package is actually this:
(defpackage "KEY-PARAM" ...)
But you can as well use:
(defpackage :key-param ...)
and
(defpackage #:key-param ...)
(here #: is a reader macro to create uninterned symbols; and this way is the preferred one, because you don't create unneeded keywords in the process).
The latter two forms will be converted to upper-case strings. So a keyword stays a keyword, and a package gets its named as string, converted from that keyword.
To sum up, keywords have the value of themselves, as well as any other symbols. The difference is that keywords don't require explicit qualification with keyword package or its explicit usage. And as other symbols they can serve as names for objects. Like, for example, you can name a function with a keyword and it will be "magically" accessible in every package :) See #Xach's blogpost for details.
There is no need for "the system" to distinguish different uses of keywords. They are just used as names. For example, imagine two plists:
(defparameter *language-scores* '(:basic 0 :common-lisp 5 :python 3))
(defparameter *price* '(:basic 100 :fancy 500))
A function yielding the score of a language:
(defun language-score (language &optional (language-scores *language-scores*))
(getf language-scores language))
The keywords, when used with language-score, designate different programming languages:
CL-USER> (language-score :common-lisp)
5
Now, what does the system do to distinguish the keywords in *language-scores* from those in *price*? Absolutely nothing. The keywords are just names, designating different things in different data structures. They are no more distinguished than homophones are in natural language – their use determines what they mean in a given context.
In the above example, nothing prevents us from using the function with a wrong context:
(language-score :basic *prices*)
100
The language did nothing to prevent us from doing this, and the keywords for the not-so-fancy programming language and the not-so-fancy product are just the same.
There are many possibilities to prevent this: Not allowing the optional argument for language-score in the first place, putting *prices* in another package without externing it, closing over a lexical binding instead of using the globally special *language-scores* while only exposing means to add and retrieve entries. Maybe just our understanding of the code base or convention are enough to prevent us from doing that. The point is: The system's distinguishing the keywords themselves is not necessary to achieve what we want to implement.
The specific keyword uses you ask about are not different: The implementation might store the bindings of keyword-arguments in an alist, a plist, a hash-table or whatever. In the case of package names, the keywords are just used as package designators and the package name as a string (in uppercase) might just be used instead. Whether the implementation converts strings to keywords, keywords to strings, or something entirely different internally doesn't really matter. What matters is just the name, and in which context it is used.
Bit of background, I'm a total lisp noob, only started a few weeks ago, but I've been developing in other langs for years. Logic no problem, lisp, problem.
I'm trying to write a macro that will define two clsql classes for me to get around a problem with the library. I'd like the classes to be named x and `x-insert`` , so within the macro I'd like the macro to compute the symbol name of x-insert, but I'm having difficulity doing this. My attempt is below, but i'm stumped on two things.
How do I get it to create the class names. If i remove the space in ,class -insert, it wont eval, which I understand, so I presume I'm missing some straightforward way to tell it to ignore the space,and create the name as a single word, and the second problem is getting it to create two classes, not one, as its only expanding the last part of the macro from what I can see using macro expand.
Perhaps I'm going about this the wrong way altogether, so feel free to kick me in the right direction.
(defmacro gen-pair (class base-slots pkey-slot base-table)
`(clsql:def-view-class ,class -insert()
(
,base-slots
)
(:base-table ,base-table)
)
`(clsql:def-view-class ,class (,class -insert)
(
,pkey-slot
)
(:base-table ,base-table)
)
)
It is difficult to begin an explanation here, since you seem to have a
whole stack of misconceptions.
First question (how to compose symbol names): Lisp macros do not
operate on text but on code. In a backquote form, ,class
evaluates to the code passed into the class parameter of the macro,
most likely a class name in this case. Writing another symbol after
that does not magically merge the symbol names; why should it? If you
want to compose a new symbol name, you have to construct it:
,(intern (string-upcase (concatenate 'string
(symbol-name class)
"-insert")))
Second question (why it seems to expand only the second part): the
contents of a defmacro form are evaluated in an implicit progn
(that is why it does not complain about an invalid number of arguments
here). The return value of the last form is the return value of the
whole defmacro form. In this case, the return value is the code
produced by that backquote form. A macro defines a function that
expands a form into a new form; you cannot expand it into two
unrelated forms. You have to produce a progn form that contains the
two forms you want to have.
Third question (why your code looks so different from what Lispers
write): do not throw around parentheses like nail clippings. There
are several Lisp style guides flying around on the net. Read them.
Wer die Form beherrscht, kann mit ihr spielen (roughly: when you
know the proper way, you can play with it).
Fourth question (how to come around the perceived limitation of
clsql): you could ask that question directly, no? What limitation do
you mean?
I've been getting my hands wet with emacs lisp, and one thing that trips me up sometimes is the dynamic scope. Is there much of a future for it? Most languages I know use static scoping (or have moved to static scoping, like Python), and probably because I know it better I tend to prefer it. Are there specific applications/instances or examples where dynamic scope is more useful?
There's a good discussion of this issue here. The most useful part that pertains to your question is:
Dynamic bindings are great for
modifying the behaviour of subsystems.
Suppose you are using a function ‘foo’
that generates output using ‘print’.
But sometimes you would like to
capture the output in a buffer of your
choosing. With dynamic binding, it’s
easy:
(let ((b (generate-new-buffer-name " *string-output*"))))
(let ((standard-output b))
(foo))
(set-buffer b)
;; do stuff with the output of foo
(kill-buffer b))
(And if you used this kind of thing a
lot, you’d encapsulate it in a macro –
but luckily it’s already been done as
‘with-output-to-temp-buffer’.)
This works because ‘foo’ uses the
dynamic binding of the name
‘standard-output’, so you can
substitute your own binding for that
name to modify the behaviour of ‘foo’
– and of all the functions that ‘foo’
calls.
In a language without dynamic binding,
you’d probably add an optional
argument to ‘foo’ to specify a buffer
and then ‘foo’ would pass that to any
calls to ‘print’. But if ‘foo’ calls
other functions which themselves call
‘print’ you’ll have to alter those
functions as well. And if ‘print’ had
another option, say ‘print-level’,
you’d have to add that as an optional
argument as well… Alternatively, you
could remember the old value of
‘standard-output’, substitute your new
value, call ‘foo’ and then restore the
old value. And remember to handle
non-local exits using ‘throw’. When
you’re through with this, you’ll see
that you’ve implemented dynamic
binding!
That said, lexical binding is IMHO much better for 99% of the cases. Note that modern Lisps are not dynamic-binding-only like Emacs lisp.
Common Lisp supports both forms of binding, though the lexical one is used much more
The Scheme specification doesn't even specify dynamic binding (only lexical one), though many implementations support both.
In addition, modern languages like Python and Ruby that were somewhat inspired by Lisp usually support lexical-binding in a straightforward way, with dynamic binding also available but less straightforward.
If you read the Emacs paper (written in 1981), there's a specific section "Language Features for Extensibility" that addresses this question. In Emacs, there's also the added scope of buffer-local (file local) variables.
I've quoted the most relevant portion below:
Formal Parameters Cannot Replace
Dynamic Scope
Some language designers believe that
dynamic binding should be avoided, and
explicit argument passing should be
used instead. Imagine that function A
binds the variable FOO, and calls the
function B, which calls the function
C, and C uses the value of FOO.
Supposedly A should pass the value as
an argument to B, which should pass it
as an argument to C.
This cannot be done in an extensible
system, however, because the author of
the system cannot know what all the
parameters will be. Imagine that the
functions A and C are part of a user
extension, while B is part of the
standard system. The variable FOO does
not exist in the standard system; it
is part of the extension. To use
explicit argument passing would
require adding a new argument to B,
which means rewriting B and everything
that calls B. In the most common case,
B is the editor command dispatcher
loop, which is called from an awful
number of places.
What's worse, C must also be passed an
additional argument. B doesn't refer
to C by name (C did not exist when B
was written). It probably finds a
pointer to C in the command dispatch
table. This means that the same call
which sometimes calls C might equally
well call any editor command
definition. So all the editing
commands must be rewritten to accept
and ignore the additional argument. By
now, none of the original system is
left!