It is a piece of data definition (comment) from the book How to Design Programs that teaches Scheme-like language and DrRacket IDE. What does #, inside [Maybe #, Letter] mean?
; A HMWord is [List-of [Maybe #, Letter]]
; A [Maybe X] is one of:
; – false
; – X
The source is here:
http://www.ccs.neu.edu/home/matthias/HtDP2e/part_4.html
Looks like a typo to me, I think it should read:
; A HMWord is [List-of [Maybe Letter]]
But then it talks about underlines, although LETTERS does not contain any underline.
So I think here false should take the role of the underline (an unknown letter).
Related
Is it possible in Common Lisp to get a symbol-name without the uppercase result?
(symbol-name 'aAbB)
;; => "AABB"
(OTHER_FUNCTION? 'aAbB)
;; => "aAbB"
I would like to use a symbol name as a string but case-sensitive.
Your symbol is actually all uppercase, because the reader already upcases it. In order to prevent that, you can either use a different readtable-case or escape the symbol, using either enclosing pipe symbols: '|aAbB| or a backslash for the next character: '\aA\bB.
There is quite a full answer on this question: Why is Common Lisp case insensitive
"The readtable objects has an attribute, readtable-case, that controls how the reader interns and evaluates the symbols read. you can setf readtable-case to :upcase(default), :downcase, :preserve, :invert.
By default, the readtable-case is set to :upcase, which causes all symbols to be converted to upcase."
I understand that both suppress evaluation of a symbol or expression. But the backtick is used for macro definitions while the apostrophe is used for symbols (among other things). What is the difference, semantically speaking, between these two notations?
Backticks allow for ,foo and ,#foo to interpolate dynamic parts into the quoted expression.
' straight up quotes everything literally.
If there are no comma parts in the expression, ` and ' can be used interchangeably.
A standard quote is a true constant literal and similar lists and list that end with the same structure can share values:
'(a b c d) ; ==> (a b c d)
A backquoted structure might not be a literal. It is evaluated as every unquote needs to be evaluated and inserted into place. This means that something like `(a ,#b ,c d) actually gets expanded to something similar to (cons 'a (append b (cons c '(d)))).
The standard is very flexible on how the implementations solves this so if you try to macroexpand the expression you get many different solutions and sometimes internal functions. The result though is well explained in the standard.
NB: Even though two separate evaluation produces different values the implementation is still free to share structure and thus in my example '(d) has the potential to be shared and if one would use mutating concatenation of the result might end up with an infinite structure.
A parallel to this is that in some algol languages you have two types of strings. One that interpolates variables and one that don't. Eg. in PHP
"Hello $var"; // ==> 'Hello Shoblade'
'Hello $var'; // ==> 'Hello $var'
On Linux with UTF-8 enabled console:
Clojure 1.6.0
user=> (def c \の)
#'user/c
user=> (str c)
"の"
user=> (def c \🍒)
RuntimeException Unsupported character: \🍒 clojure.lang.Util.runtimeException (Util.java:221)
RuntimeException Unmatched delimiter: ) clojure.lang.Util.runtimeException (Util.java:221)
I was hoping to have an emoji-rich Clojure application with little effort, but it appears I will be looking up and typing in emoji codes? Or am I missing something obvious here? 😞
Java represents Unicode characters in UTF-16. The emoji characters are "supplementary characters" and have a codepoint that cannot be represented in 16 bits.
http://www.oracle.com/technetwork/articles/javase/supplementary-142654.html
In essence, supplementary characters are represented not as chars but as ints and there are special apis for dealing with them.
One way is with (Character/toChars 128516) - this returns a char array that you can convert to a string to print: (apply str (Character/toChars 128516)). Or you can create a String from an array of codepoint ints directly with (String. (int-array [128516]) 0 1). Depending on all the various things between Java/Clojure and your eyeballs, that may or may not do what you want.
The format api supports supplementary characters so that may be easiest, however it takes an int so you'll need a cast: (format "Smile! %c" (int 128516)).
Thanks to Clojure’s extensible reader tags, you can create Unicode literals quite easily yourself.
We already know that not all of Unicode can be represented as char literals; that the preferred representation of Unicode characters on the JVM is int; and that a string literal can hold any Unicode character in a way that’s also convenient for humans to read.
So, a tagged literal #u "🍒" that reads as an int would make an excellent Unicode character literal!
Set up a reader function for the new tagged literal in *data-readers*:
(defn read-codepoint
[^String s]
{:pre [(= 1 (.codePointCount s 0 (.length s)))]}
(.codePointAt s 0))
(set! *data-readers* (assoc *data-readers* 'u #'read-codepoint))
With that in place, the reader reads such literals as code point integers:
#u"🍒" ; => 127826
(Character/getName #u"🍒") ; => "CHERRIES"
‘Reader tags without namespace qualifiers are reserved for Clojure’, says the documentation … #u is short but perhaps not the most responsible choice.
When I read some elisp code, I found something like:
(\,(* 2 \#1))
\,(format "%s %s id%d %s" \1 \2 (+1 \#) \3)
#'(bla bla)
What does the symbol like "\,", "#", "#'" mean? Which session should I look into for those kind of things?
\, is special in replacements when using query-replace-regexp. It means "evaluate the following elisp expression, and use the resulting value in the replacement".
n.b. It's not special elsewhere (that I'm aware of), so that should be the usage you've seen.
\# is also special in the replacement string, and is substituted with the number of replacements made thus far. (i.e. an incrementing counter).
\#N (where N is a number) is a variant of \N which treats the group in question as a number rather than a string, which is useful when the expression you're evaluating requires a number.
So (\,(* 2 \#1)) would be a replacement which evaluates the expression (* 2 \#1), multiplying the number matched by the first group of the regexp by 2 to produce some value N, such that the final replacement is (N).
You can find these detailed in the manual.
C-hig (emacs) RET followed by a search for the syntax in question. e.g. C-s \, with a repeated C-s if the search fails (as it will) to find a match in the subsequent nodes.
#'... is short-hand for (function ...) which is a variant of '... / (quote...) which indicates that the quoted object is a function.
As this is elisp syntax, you find it in the elisp manual:
C-hig (elisp) RET
You can either use C-s #' or in this case it's indexed, so I #' RET also works.
(In general check the index first, and then use isearch.)
For info on backquotes, see http://www.gnu.org/software/emacs/manual/html_node/elisp/Backquote.html.
# starts the reader syntax, for instance #' is a reader alias for function.
For more info see http://definitelyaplug.b0.cx/post/emacs-reader/
The #' is a short hand for using functions, for more details see here: http://www.gnu.org/software/emacs/manual/html_node/elisp/Anonymous-Functions.html
Backslash \ has two functions: it quotes the special characters (including ‘\’), and it introduces additional special constructs. More here: https://www.gnu.org/software/emacs/manual/html_node/emacs/Regexps.html#Regexps
For the love of the almighty I have yet to understand the purpose of the symbol 'iamasymbol. I understand numbers, booleans, strings... variables. But symbols are just too much for my little imperative-thinking mind to take. What exactly do I use them for? How are they supposed to be used in a program? My grasp of this concept is just fail.
In Scheme and Racket, a symbol is like an immutable string that happens to be interned so that symbols can be compared with eq? (fast, essentially pointer comparison). Symbols and strings are separate data types.
One use for symbols is lightweight enumerations. For example, one might say a direction is either 'north, 'south, 'east, or 'west. You could of course use strings for the same purpose, but it would be slightly less efficient. Using numbers would be a bad idea; represent information in as obvious and transparent a manner as possible.
For another example, SXML is a representation of XML using lists, symbols, and strings. In particular, strings represent character data and symbols represent element names. Thus the XML <em>hello world</em> would be represented by the value (list 'em "hello world"), which can be more compactly written '(em "hello world").
Another use for symbols is as keys. For example, you could implement a method table as a dictionary mapping symbols to implementation functions. To call a method, you look up the symbol that corresponds to the method name. Lisp/Scheme/Racket makes that really easy, because the language already has a built-in correspondence between identifiers (part of the language's syntax) and symbols (values in the language). That correspondence makes it easy to support macros, which implement user-defined syntactic extensions to the language. For example, one could implement a class system as a macro library, using the implicit correspondence between "method names" (a syntactic notion defined by the class system) and symbols:
(send obj meth arg1 arg2)
=>
(apply (lookup-method obj 'meth) obj (list arg1 arg2))
(In other Lisps, what I've said is mostly truish, but there are additional things to know about, like packages and function vs variable slots, IIRC.)
A symbol is an object with a simple string representation that (by default) is guaranteed to be interned; i.e., any two symbols that are written the same are the same object in memory (reference equality).
Why do Lisps have symbols? Well, it's largely an artifact of the fact that Lisps embed their own syntax as a data type of the language. Compilers and interpreters use symbols to represent identifiers in a program; since Lisp allows you to represent a program's syntax as data, it provides symbols because they're part of the representation.
What are they useful apart from that? Well, a few things:
Lisp is commonly used to implement embedded domain-specific languages. Many of the techniques used for that come from the compiler world, so symbols are an useful tool here.
Macros in Common Lisp usually involve dealing with symbols in more detail than this answer provides. (Though in particular, generation of unique identifiers for macro expansions requires being able to generate a symbol that's guaranteed never to be equal to any other.)
Fixed enumeration types are better implemented as symbols than strings, because symbols can be compared by reference equality.
There are many data structures you can construct where you can get a performance benefit from using symbols and reference equality.
Symbols in lisp are human-readable identifiers. They are all singletons. So if you declare 'foo somewhere in your code and then use 'foo again, it will point to the same place in memory.
Sample use: different symbols can represent different pieces on a chessboard.
From Structure and Interpretation of Computer Programs Second Edition by Harold Abelson and Gerald Jay Sussman 1996:
In order to manipulate symbols we need a new element in our language:
the ability to quote a data object. Suppose we want to construct the list
(a b). We can’t accomplish this with (list a b), because this expression
constructs a list of the values of a and b rather than the symbols themselves.
This issue is well known in the context of natural languages, where words
and sentences may be regarded either as semantic entities or as character
strings (syntactic entities). The common practice in natural languages is to use quotation marks to indicate that a word or a sentence is to be treated
literally as a string of characters. For instance, the first letter of “John” is
clearly “J.” If we tell somebody “say your name aloud,” we expect to hear
that person’s name. However, if we tell somebody “say ‘your name’ aloud,”
we expect to hear the words “your name.” Note that we are forced to nest
quotation marks to describe what somebody else might say.
We can follow this same practice to identify lists and symbols that are
to be treated as data objects rather than as expressions to be evaluated.
However, our format for quoting differs from that of natural languages in
that we place a quotation mark (traditionally, the single quote symbol ’)
only at the beginning of the object to be quoted. We can get away with this in Scheme syntax because we rely on blanks and parentheses to delimit
objects. Thus, the meaning of the single quote character is to quote the
next object.
Now we can distinguish between symbols and their values:
(define a 1)
(define b 2)
(list a b)
(1 2)
(list ’a ’b)
(a b)
(list ’a b)
(a 2)
Lists containing symbols can look just like the expressions of our language:
(* (+ 23 45) (+ x 9))
(define (fact n) (if (= n 1) 1 (* n (fact (- n 1)))))
Example: Symbolic Differentiation
A symbol is just a special name for a value. The value could be anything, but the symbol is used to refer to the same value every time, and this sort of thing is used for fast comparisons. As you say you are imperative-thinking, they are like numerical constants in C, and this is how they are usually implemented (internally stored numbers).
To illustrate the point made by Luis Casillas, it might be useful to observe how symbols eval differently than strings.
The example below is for mit-scheme (Release 10.1.10). For convenience, I use this function as eval:
(define my-eval (lambda (x) (eval x (scheme-report-environment 5))))
A symbol can easily evaluate to the value or function it names:
(define s 2) ;Value: s
(my-eval "s") ;Value: "s"
(my-eval s) ;Value: 2
(define a '+) ;Value: a
(define b "+") ;Value: b
(my-eval a) ;Value: #[arity-dispatched-procedure 12]
(my-eval b) ;Value: "+"
((my-eval a) 2 3) ;Value: 5
((my-eval b) 2 3) ;ERROR: The object "+" is not applicable.