I finally started learning functional languages (emacs lisp) and it makes explicit distinction between functions and special forms such as flow control , for example if.
Is there a fundamental/theoretical reason why special forms are distinct from functions? do any languages provide functional if?
Thanks
With eager evaluation the distinction is required, languages with lazy evaluation (i.e. Haskell) if et al. can be functions.
Eager evaluation: The function's arguments are evaluated before calling the function, and only the results are passed to the function.
Lazy evaluation: A function's arguments evaluated if and only if they are accessed.
If if was a normal function, then both its arguments—the then form and the else form—would both be evaluated before calling the if function, because that's the rule of function evaluation: evaluate all arguments to produce values, then provide that sequence of values as arguments to the function designated by the first symbol in the list.
Instead, with if what you want to do is evaluate exactly one of the then form and else form, not both. In order to suppress evaluation of one or the other, you need either a macro or a special form.
In languages like Emacs Lisp and Common Lisp, special forms are built-in language constructs. They have different evaluation rules that normal function calls. For normal function calls all arguments are evaluated. So, you can't write an IF as a normal function - the condition determines which clause gets evaluated. Also usually you can't write your own special forms - in Common Lisp there is no language construct for defining a special form (though individual implementations must have implemented the existing ones somehow. This leads to macros. With macros you can write a syntactic transformation that transforms one expression into another one. To be able to write IF as a macro, you need to have another conditional form, which you can use for the transformed code. Lisp provides conditionals as basic constructs. Let's assume COND is such a basic construct, then you could expand IF into a usage of COND.
MY-IF as a macro in Common Lisp:
(defmacro my-if (condition true-clause false-clause)
`(cond (,condition ,true-clause)
(t ,false-clause)))
So
(my-if (foo-p) 'one 'two)
gets expanded into
(cond ((foo-p) 'one)
(t 'two))
For completeness: there are no special forms in the Pico language for example, and if is a primitive function while Pico is inspired by Scheme and has eager evaluation by default.
In Scheme you could write
(define (true t f)
(t))
(define (false t f)
(f))
(define (function_if c t e)
(c t e))
and then
(function_if true (lambda () 'true) (lambda () 'false))
==> true
What makes this manageable in Pico is that you can define functional parameters that take functional arguments that are "automatically" delayed. This means that you don't have to do the wrapping inside lambdas yourself. Pico therefore has eager evaluation but with lazy evaluation on demand, bypassing the need for special forms.
So, in Scheme syntax with functional parameters you can encode booleans as:
(define (true (t) (f))
(t))
(define (false (t) (f))
(f))
Then function if becomes:
(define (function_if c (t) (e))
(c (t) (e)))
and
(function_if true 'true 'false)
==> true
As another example, the definition of the function and is (define (and p (q)) (p (q) false)).
Similarly you can define or, not, while, for, ... as functions, using the above encoding of booleans.
Short answer: No.
Long(er) answer: (if ...) requires that you control the evaluation order of the arguments. Lisp, being an eager language cannot do this in a function.
Workaround: do it in a macro:
(defmacro _if (cnd true false)
(let ( (gcond (gensym))
(gresp (gensym)))
`(let ( (,gcond ,cnd) ;`#quotes
(,gresp nil))
(and ,gcond (setf ,gresp (multiple-value-list ,true)))
(and (not ,gcond) (setf ,gresp (multiple-value-list ,false)))
(values-list ,gresp))))
For example:
[dsm#localhost:~]$ clisp -q
[1]> (defmacro _if (cnd true false)
(let ( (gcond (gensym))
(gresp (gensym)))
`(let ( (,gcond ,cnd) ;`#quotes
(,gresp nil))
(and ,gcond (setf ,gresp (multiple-value-list ,true)))
(and (not ,gcond) (setf ,gresp (multiple-value-list ,false)))
(values-list ,gresp))))
_IF
[2]> (_if (= 1 1) (+ 2 3) "bar")
5
[3]> (_if (= 1 2) (+ 2 3) "bar")
"bar"
[4]>
In Scala it's possible to model if with correct side-effect evaluation using call-by-name arguments.
def If[A](cond : Boolean, truePart : => A, falsePart : => A) = if (cond) truePart else falsePart
These feature can be used to model lots of new control structures as well.
IF could be a function in a functional language having call-by-name semantics (lazy evaluation), as in Lambda Calculus or Algol. In fact that is, I think, at the heart of the relationship between Turing Machines and Lambda Calculus as equivalent foundations for computing. However, in languages having side-effects (like assignments to variables) it is not much use, because when things happen is important.
Related
I'm studying Clojure, and I've read that in Clojure a function definition is just data, i.e. parameters vector is just an ordinary vector. If that's the case, why can I do this
(def add (fn [a b]
(+ a b)))
but not this
(def vector-of-symbols [a b])
?
I know I normally would have to escape symbols like this:
(def vector-of-symbols [`a `b])
but why don't I have to do it in fn/defn? I assume this is due to fn/defn being macros. I tried examining their source, but they are too advanced for me so far. My attempts to recreate defn also fail, and I'm not sure why (I took example from a tutorial):
(defmacro defn2 [name param & body]
`(def ~name (fn ~param ~#body)))
(defn2 add [a b] (+ a b)) ;;I get "Use of undeclared Var app.core/defn2"
Can someone please explain, how exactly does Clojure turn data structures, especially symbols, into code? And what am I missing about the macro example?
Update Apparently, macro does not work because my project is actually in Clojurescript (in Clojure it does work). I did not think it matters, but as I progress - I discover more and more things that somehow don't work for me in with Clojurescript.
Update 2 This helps: https://www.clojurescript.org/about/differences
A function is a first-class citizen as other data in Clojure.
To define a vector you use (vector ...) or reader has syntaxic sugar [...], for a list it's (list ...) or '(...) the quote not to evaluate the list as a function call, for a set (set ...) or #{...}.
So the factory function for a function is fn (in fact fn*, that comes from Java core of Clojure, fn is a series of macros to manage to destructure and all).
(fn args body)
is a function call that returns a function, where args is a vector of argument(s) event. empty and body is a series of Clojure expressions to be evaluated with args bind to the environment. If nothing is to be evaluated it returns nil. There is also a syntactic sugar #(...) with %x as argument x and % as argument 1.
(fn ...) return a value that is a function. So
(def my-super-function (fn [a b c d] (println "coucou") (+ a b c d)))
binds the symbol my-super-function with the anonymous function returned by (fn [a b c d] (println "coucou") (+ a b c d)).
(def my_vector [1 2 3])
binds the symbol my_vector with the vector [1 2 3]
List of learning resources: https://github.com/io-tupelo/clj-template#documentation
As #jas said, your defn2 macro looks fine.
The main point is that macros are an advanced feature that one almost never needs. A macro is equivalent to a compiler extension, and that is almost never the best solution to a problem. Also keep in mind that functions can do some things macros can't.
Another point: the syntax-quote (aka backquote) ` is very different from a single quote '. In your example you want the single quote for ['a 'b]. Even better would be to quote the entire vector form '[a b].
As to your primary question, it is poorly explained how source-file text is converted into code. This is a 2-step process. The Clojure Reader consumes text string data (from a file or a literal string) and produces data structures like lists, vectors, strings, numbers, symbols. The Clojure compiler takes these data structures as input and produces java byte code that can be executed.
It is confusing because, when printed, one can't tell the difference between the text representation of a vector [1 2 3] and the text string that is input to the reader [1 2 3]. Ideally it would be color-coded or something. This problem doesn't exist in Java, etc since they don't have macros and hence there is no confusion between the source code (text) and the data structures used by a macro (not text).
For a more detailed answer on creating macros in Clojure, please see this answer.
Some languages like Racket don't need a return statement to give the caller back a value. For example:
(define (myfunc)
(1))
Why do other languages such as C# have an explicit return keyword? Is there a real use for this keyword, or is it just for more clarity?
This is typical of functional languages where the function returns the last evaluated expression.
In an imperative context, this is an impractical restriction, since it is common to return early, for example when seeing if an array contains a certain element
contains(array, element) {
for(el in array) {
if(el == element) return true;
}
return false;
}
whereas in a functional context you would accomplish this with recursion
(define (contains array element)
(cond
((empty? array) false)
((eq (head array) element) true)
(else (contains (tail array) element))))
Your question is somewhat related to the difference between a function and a procedure.
Functional programing languages support only functions because they promote the paradigm of writing your code in a "functional way" ,meaning, no side effects, which means that you should always return something (the result).
That doesn't mean that you can't write non-functional code in a functional-programing language, Scheme, for example, has !set. That said - it is not encouraged to use it because of its a non-functional pattern.
For the following code:
(foldl and #t '(#t #f))
Racket returns:
and: bad syntax in: and
I know and is not a function. And I can circumvent this problem using lambda:
(foldl (lambda (a b) (and a b)) #t '(#t #f))
I have 2 questions here:
and is not a function. Then what is it? Is it a macro?
My solution using lambda seems ugly. Is there a better way to solve this problem?
Thank you.
It is a conditional syntactic form, or it might be implemented as a macro that expands to some core syntax form, which is treated as a special case by the compiler/interpreter.
The list there in Racket's docs includes if as a special form but doesn't include and, so the latter most probably is implemented in terms of the former. But R5RS does list and as a syntactic keyword. So, best we can say, it's either a special syntax, or a macro.
It is easy to re-write any and form (and a b c ...) as an if form, (if a (if b (if c #t #f) #f) #f).
lambda is fine by me, but you can also use every from SRFI-1 (or Racket's andmap):
(every identity '(#t #f))
should return #f.
edit: except, as Joshua Taylor points out, calling your lambda through a function like foldl does not short-circuit. Which defeats the purpose to calling the and in the first place.
Another thing is, in Racket's foldl the last argument to lambda is the one that receives the previous result in the chain of applications; so the implementation should really be
(foldl (lambda (a b) (and b a)) #t '(#t #f))
I am trying to determine whether a given argument within a macro is a function, something like
(defmacro call-special? [a b]
(if (ifn? a)
`(~a ~b)
`(-> ~b ~a)))
So that the following two calls would both generate "Hello World"
(call-special #(println % " World") "Hello")
(call-special (println " World") "Hello")
However, I can't figure out how to convert "a" into something that ifn? can understand. Any help is appreciated.
You might want to ask yourself why you want to define call-special? in this way. It doesn't seem particularly useful and doesn't even save you any typing - do you really need a macro to do this?
Having said that, if you are determined to make it work then one option would be to look inside a and see if it is a function definition:
(defmacro call-special? [a b]
(if (#{'fn 'fn*} (first a))
`(~a ~b)
`(-> ~b ~a)))
This works because #() function literals are expanded into a form as follows:
(macroexpand `#(println % " World"))
=> (fn* [p1__2609__2610__auto__]
(clojure.core/println p1__2609__2610__auto__ " World"))
I still think this solution is rather ugly and prone to failure once you start doing more complicated things (e.g. using nested macros to generate your functions)
First, a couple of points:
Macros are simply functions that receive as input [literals, symbols, or collections of literals and symbols], and output [literals, symbols, or collections of literals and symbols]. Arguments are never functions, so you could never directly check the function the symbol maps to.
(call-special #(println % " World") "Hello") contains reader macro code. Since reader macros are executed before regular macros, you should expand this before doing any more analysis. Do this by applying (read-string "(call-special #(println % \" World\") \"Hello\")") which becomes (call-special (fn* [p1__417#] (println p1__417# "world")) "Hello").
While generally speaking, it's not obvious when you would want to use something when you should probably use alternative methods, here's how I would approach it.
You'll need to call macroexpand-all on a. If the code eventually becomes a (fn*) form, then it is guaranteed to be a function. Then you can safely emit (~a ~b). If it macroexpands to eventually be a symbol, you can also emit (~a ~b). If the symbol wasn't a function, then an error would throw at runtime. Lastly, if it macroexpands into a list (a function call or special form call), like (println ...), then you can emit code that uses the thread macro ->.
You can also cover the cases such as when the form macroexpands into a data structure, but you haven't specified the desired behavior.
a in your macro is just a clojure list data structure (it is not a function yet). So basically you need to check whether the data structure a will result is a function or not when it is evaluated, which can be done like show below:
(defmacro call-special? [a b]
(if (or (= (first a) 'fn) (= (first a) 'fn*))
`(~a ~b)
`(-> ~b ~a)))
By checking whether the first element of the a is symbol fn* or fn
which is used to create functions.
This macro will only work for 2 cases: either you pass it a anonymous function or an expression.
I am writing a simple member function that returns the value associated with that member. The first line of the conditional (to test if the first value is the one we're looking for) works fine, but the recursive part (to test parts of the list that are further along) returns an error every time.
Here is my function:
(defun mem (prop L)
(cond
((eq (caar L) prop) (print cadar L)))
(t (mem (prop (cdr L)))))) // error is on this line!
So, if I enter
(mem `i `((i 1) (j 2)))
it correctly returns 1. However, if I enter
(mem `j `((i 1) (j 2)))
it returns an error that "function prop is undefined."
How do I let the program know that prop isn't a function, but is just an input parameter?
This is my first lisp program, so I'm assuming the answer is incredibly simple, but I have tried many things and have yet to succeed.
The problem in particular is the snippet: (prop (cdr L)). This tries to call prop, passing it (cdr L). To pass it as an argument to mem, simply leave out the extra parentheses: (mem prop (cdr L)).
If you're having a problem figuring out where to put parentheses in general, note that Lisp syntax is very similar to mathematical syntax for functions, except the function goes on the inside of the parentheses (and you use spaces instead of commas, but that isn't a problem here). For example, written in mathematical notation, you have: mem(prop(cdr(L))), instead of mem(prop, cdr(L)).
Other Issues
It looks like you have an extra parentheses after the first branch of the cond, which ends it early. Use an editor/IDE with parentheses matching to catch this sort of error. Emacs does it, as does DrRacket. The former can be used with Common LISP via SLIME (search the web for set-up instructions for your platform), while the latter readily supports Scheme.
The semicolon (";") is the comment character used in most LISPs.
print isn't necessary for this function, and (depending on its behavior) may be incorrect. In some LISPs (e.g. Common LISP), it returns its argument, but in others it may return nil or the instance of some void type.
Quasiquote (aka backquote, backtick) is also unnecessary. Quasiquote lets you quote some symbols, while others (those prefixed with a comma) are replaced with their values. Since you don't need substitution in the quoted expressions, a plain quote will work: (mem 'j '((i 1) (j 2))).
For readability, insert more whitespace between closing and opening parentheses.