I stumbled again in the good old '12' + 2 = '122'
I wanted to deeply understand what happens here, so my first thesis was that
Maybe Javascript casts the right operand to the type of the first one and
then operates, like so: '12' + String(2) = '122' all good...
But no, because 12 + '2' = '122' too; So the engine's magic is clearly favoring to concat over casting to number.
My second thesis was then
Maybe the engine enumerates all operands and looks for an "operator override", similar to C#? And then favor executing that over doing the self-magic thing?
My confusion got even weirder when I realized that also '5' * '8' = 40, it casts both operands to Number and does the operation.
The only way I could possibly really understand that was to read the V8 code directly from GitHub
The farther I could track down was at v8/src/parsing/parser-base.h line 2865
// We have a "normal" binary operation.
x = factory()->NewBinaryOperation(op, x, y, pos);
if (op == Token::OR || op == Token::AND) {
impl()->RecordBinaryOperationSourceRange(x, right_range);
}
From here I got lost, because I couldn't find where this factory() is coming from.
Long story short, where does the JavaScript "type Magic" come from in the V8 Engine Source code?
V8 developer here.
There are several fast paths for various cases of addition and other operations in V8. If you want to study a canonical (slow, but complete) version, you can look for Object::Add in src/objects.cc.
That said, the source of truth here is not any given engine's implementation, but the JavaScript specification. What the + operator is supposed to do is defined here: https://tc39.github.io/ecma262/#sec-addition-operator-plus.
Any engine's implementation either does precisely that, or something that from the outside is indistinguishable from that -- otherwise it's a bug. It's not a coincidence that the implementation of Object::Add reads almost exactly like the spec ;-)
I am concerned about writing self-modifying code in Ruby. And by self-modifying, I mean being able to write functions that take a code block as an input value, and output another code block based on this. (I am not asking about basics such as redefining methods at runtime.)
What I might want to do is, for example, having the following block,
_x_ = lambda { |a, b, c, d| b + c }
one can notice that arguments a and d are not used in the body at all, so I would like a function eg. #strip to remove them,
x = _x_.strip
which should produce same result as writing:
x = lambda { |b, c| b + c }
Now in Lisp, this would be easy, since Lisp code is easily manipulable data. But I do not know how to manipulate Ruby code. I can parse it eg. by
RubyVM::InstructionSequence.disassemble( x )
But how, based on this, do I write a modified block? Other examples of what I would want to do are are eg.
y = lambda { A + B }
y.deconstantize
# should give block same as saying
lambda { |_A, _B| _A + _B }
So far, in Ruby, I have never encountered a situation where I had to concede that something is not possible. But this time, gut feeling tells me that I might have encountered the fundamental weakness of beautifully structured code vs. code with little syntax to speak about (which would be Lisp). Please enlighten me.
Detecting whether a block variable is used or not is a complicated task, and you seem to be saying that you can do that by using RubyVM. So the question seems to be asking how to change the arity of the code.
If you have:
_x_ = ->a, b, c, d{b + c}
and suppose you were able to use RubyVM and come to know that a and d are not used, so you want to create
x = ->b, c{b + c}
out of _x_. Then, that is simple:
x = ->b, c{_x_.call(nil, b, c, nil)}
Boris do you necessarily have to rely on Ruby to begin with here?
Why not just create your own situation-specific language that the chemists can use just for the purpose to express their formulas in the most convenient way. Then you create a simple parser and compiler for this "chemical expression language".
What I mean is this parser and compiler will parse and compile the expressions the chemists write in their Ruby code. Then you could have:
ChemicalReaction.new(..., "[ATP] * [GDP] * NDPK_constant")
Voila: ultimate flexibility.
That's the approach I would take if usability is your main concern. Already writing out "lambda" seems like an unnecessarily cumbersome thing to me here, if all you want to do is express some domain-specific formula in the most compact way possible.
I would like to declare a data type in Mercury that can have a variable number of values and names. For instance :
type goal ---> pick; give; come.
has three variables/values.
I want something like:
type myplayer ---> de value declaration here.
That's the number of variables are not restricted or fixed.
So I can use myplayer to declare values/variables like v1, v2, v3 and v4. The second time I can use it to declare something like: a, b, c, d, e, z, aa, ab and az.
The number of the values are not restricted and the names are also not fixed.
I am new in Mercury.
As others have said, this is simply impossible in Mercury - which is deliberate.
What you might want though, if you want a type that expresses: v1 v2 v3... etc is:
:- type my_type
----> v(int).
:- func name(my_type) = string.
name(v(Num)) = formst("v%d", [i(Num)]).
The type expresses v of all integers, and the function name can be used to 'pretty-print' values of this type.
What you directly ask for, simply cannot be done. Given
:- type question
---> truth(string, bool)
; blank(string, string)
; multiple(string, string, list(string)).
additional kinds of questions can only be added by extending this type where it is defined, and recompiling the module - and making a lot of other changes, too, as previously deterministic code like
answer(truth(_, yes)) = "true".
answer(truth(_, no)) = "false".
answer(blank(_, A)) = A.
answer(multiple(_, A, _)) = A.
would fail when given your new question type. Being told at compile-time where you've failed to update your program to reflect the addition of a "pick-all-the-right-answers" type of question is a good part of the reason you have a question type at all, instead of say lists of strings [["Are foxes pretty?", "true"], ["Green foxes are ____", "adorable!", "fake", "evidence of animal cruelty"]] for your question bank.
What you ask for cannot be done. However, what you actually want to do -- the end to which you thought 'variable numbers of types' would be an helpful means -- can surely be accomplished in some other way. I can't tell what way that is, as I can't tell why you wanted to do this from your question. Maybe you'd benefit from reading over discriminated unions or typeclasses in the language reference.
As far as I understand this question. You want some Prolog-like behavior. I.e without typed predicates. In statically typed system you always can achieve such behavior by handling that by yourself. A lot of time ago I saw such example in Turbo Prolog (they implemented ISO prolog in terms of Turbo/Visual Prolog).
Consider something like (I'm not sure it is correct):
:- type any_type ---> atom_value(string)
; number_value(int)
; struct_value(any_type, list(any_type)).
guess(atom_value("v1")).
guess(atom_value("a")).
guess(atom_value("aa")).
guess(number_value(42)).
guess(struct_value(atom_value("pair"), [number_value(3), number_value(4)])).
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Closed 10 years ago.
When have you run into syntax that might be dated, never used or just plain obfuscated that you couldn't understand for the life of you.
For example, I never knew that comma is an actual operator in C. So when I saw the code
if(Foo(), Bar())
I just about blew a gasket trying to figure out what was going on there.
I'm curious what little never-dusted corners might exist in other languages.
C++'s syntax for a default constructor on a local variable. At first I wrote the following.
Student student(); // error
Student student("foo"); // compiles
This lead me to about an hour of reading through a cryptic C++ error message. Eventually a non-C++ newbie dropped by, laughed and pointed out my mistake.
Student student;
This is always jarring:
std::vector <std::vector <int> >
^
mandatory space.
When using the System.DirectoryServices name space to bind to an ADAM (Active Directory Application Mode; now called AD LDS, I think), I lost an entire day trying to debug this simple code:
DirectoryEntry rootDSE = new DirectoryEntry(
"ldap://192.168.10.78:50000/RootDSE",
login,
password,
AuthenticationTypes.None);
When I ran the code, I kept getting a COMException with error 0x80005000, which helpfully mapped to "Unknown error."
I could use the login and password and bind to the port via ADSI Edit. But this simple line of code didn't work. Bizarre firewall permission? Something screwed in configuration? Some COM object not registered correctly? Why on earth wasn't it working?
The answer? It's LDAP://, not ldap://.
And this is why we drink.
C++
class Foo
{
// Lots of stuff here.
} bar;
The declaration of bar is VERY difficult to see. More commonly found in C, but especially annoying in C++.
Perl's syntax caused me a bad day a while ago:
%table = {
foo => 1,
bar => 2
};
Without proper warnings (which are unavailable on the platform I was using), this creates a one-element hash with a key as the given hash reference and value undef. Note the subtle use of {}, which creates a new hash reference, and not (), which is an array used to populate the %table hash.
I was shocked Python's quasi-ternary operator wasn't a syntax error the first time I saw it:
X if Y else Z
This is stupid and common, but this syntax:
if ( x = y ) {
// do something
}
Has caught me about three times in the past year in a couple of different languages. I really like the R language's convention of using <- for assignment, like this:
x <- y
If the x = y syntax were made to mean x == y, and x <- y to mean assignment, my brain would make a smoother transition to and from math and programming.
C/C++'s bitvector syntax. The worst part about this is trying to google for it simply based on the syntax.
struct C {
unsigned int v1 : 12;
unsigned int v2 : 1;
};
C#'s ?? operator threw me for a loop the first time I saw it. Essentially it will return the LHS if it's non-null and the RHS if the LHS is null.
object bar = null;
object foo = bar ?? new Student(); // gets new Student()
Powershell's function calling semantics
function foo() {
params ($count, $name);
...
}
foo (5, "name")
For the non powershellers out there. This will work but not how you expect it to. It actually creates an array and passes it as the first argument. The second argument has no explicit value. The correct version is
foo 5 "name"
The first time I saw a function pointer in C++ I was confused. Worse, because the syntax has no key words, it was really hard to look up. What exactly does one type into a search engine for this?
int (*Foo)(float, char, char);
I ended up having to ask the local C++ guru what it was.
VB's (yeah yeah, I have to use it) "And" keyword - as in:
If Object IsNot Nothing And Object.Property Then
See that Object.Property reference, after I've made sure the object isn't NULL? Well, VB's "And" keyword * does * not * block * further * evaluation and so the code will fail.
VB does have, however, another keyword - AndAlso:
If Object IsNot Nothing AndAlso Object.Property Then
That will work as you'd expect and not explode when run.
I was once very confused by some C++ code that declared a reference to a local variable, but never used it. Something like
MyLock &foo;
(Cut me some slack on the syntax, I haven't done C++ in nearly 8 years)
Taking that seemingly unused variable out made the program start dying in obscure ways seemingly unrelated to this "unused" variable. So I did some digging, and found out that the default ctor for that class grabbed a thread lock, and the dtor released it. This variable was guarding the code against simultaneous updates without seemingly doing anything.
Javascript: This syntax ...
for(i in someArray)
... is for looping through arrays, or so I thought. Everything worked fine until another team member dropped in MooTools, and then all my loops were broken because the for(i in ...) syntax also goes over extra methods that have been added to the array object.
Had to translate some scientific code from old FORTRAN to C. A few things that ruined my day(s):
Punch-card indentation. The first 6 characters of every line were reserved for control characters, goto labels, comments, etc:
^^^^^^[code starts here]
c [commented line]
Goto-style numbering for loops (coupled with 6 space indentation):
do 20, i=0,10
do 10, j=0,10
do_stuff(i,j)
10 continue
20 continue
Now imagine there are multiple nested loops (i.e., do 20 to do 30) which have no differentiating indentation to know what context you are in. Oh, and the terminating statements are hundreds of lines away.
Format statement, again using goto labels. The code wrote to files (helpfully referred to by numbers 1,2,etc). To write the values of a,b,c to file we had:
write (1,51) a,b,c
So this writes a,b,c to file 1 using a format statement at the line marked with label 51:
51 format (f10.3,f10.3,f10.3)
These format lines were hundreds of lines away from where they were called. This was complicated by the author's decision to print newlines using:
write (1,51) [nothing here]
I am reliably informed by a lecturer in the group that I got off easy.
C's comma operator doesn't seem very obscure to me: I see it all the time, and if I hadn't, I could just look up "comma" in the index of K&R.
Now, trigraphs are another matter...
void main() { printf("wat??!\n"); } // doesn't print "wat??!"
Wikipedia has some great examples, from the genuinely confusing:
// Will the next line be executed????????????????/
a++;
to the bizarrely valid:
/??/
* A comment *??/
/
And don't even get me started on digraphs. I would be surprised if there's somebody here who can fully explain C's digraphs from memory. Quick, what digraphs does C have, and how do they differ from trigraphs in parsing?
Syntax like this in C++ with /clr enabled. Trying to create a Managed Dictionary object in C++.
gcroot<Dictionary<System::String^, MyObj^>^> m_myObjs;
An oldie:
In PL/1 there are no reserved words, so you can define variables, methods, etc. with the same name as the language keywords.
This can be a valid line of code:
IF ELSE THEN IF ELSE THEN
(Where ELSE is a boolean, and IF and THEN are functions, obviously.)
Iif(condition, expression, expression) is a function call, not an operator.
Both sides of the conditional are ALWAYS evaluated.
It always ruines my day if I have to read/write some kind of Polish notation as used in a lot of HP calculators...
PHP's ternary operator associates left to right. This caused me much anguish one day when I was learning PHP. For the previous 10 years I had been programming in C/C++ in which the ternary operator associates right to left.
I am still a little curious as to why the designers of PHP chose to do that when, in many other respects, the syntax of PHP matches that C/C++ fairly closely.
EDIT: nowadays I only work with PHP under duress.
Not really obscure, but whenever I code too much in one language, and go back to another, I start messing up the syntax of the latter. I always chuckle at myself when I realize that "#if" in C is not a comment (but rather something far more deadly), and that lines in Python do not need to end in a semicolon.
While performing maintentnace on a bit of C++ code I once spotted that someone had done something like this:
for (i=0; i<10; i++)
{
MyNumber += 1;
}
Yes, they had a loop to add 1 to a number 10 times.
Why did it ruin my day? The perpetrator had long since left, and I was having to bug fix their module. I thought that if they were doing something like this, goodness knows what else I was going to encounter!
AT&T assembler syntax >:(
This counter-intuitive, obscure syntax has ruined many of my days, for example, the simple Intel syntax assembly instruction:
mov dword es:[ebp-5], 1 /* Cool, put the value 1 into the
* location of ebp minus five.
* this is so obvious and readable, and hard to mistake
* for anything else */
translates into this in AT&T syntax
movl $1, %es:-4(%ebp) /* huh? what's "l"? 4 bytes? 8 bytes? arch specific??
* wait, why are we moving 1 into -4 times ebp?
* or is this moving -4 * ebp into memory at address 0x01?
* oh wait, YES, I magically know that this is
* really setting 4 bytes at ebp-5 to 1!
More...
mov dword [foo + eax*4], 123 /* Intel */
mov $123, foo(, %eax, 4) /* AT&T, looks like a function call...
* there's no way in hell I'd know what this does
* without reading a full manual on this syntax */
And one of my favorites.
It's as if they took the opcode encoding scheme and tried to incorporate it into the programming syntax (read: scale/index/base), but also tried to add a layer of abstraction on the data types, and merge that abstraction into the opcode names to cause even more confusion. I don't see how anyone can program seriously with this.
In a scripting language (Concordance Programming Language) for stand alone database software (Concordance) used for litigation document review, arrays were 0 indexed while (some) string functions were 1 indexed. I haven't touched it since.
This. I had my run in with it more then once.
GNU extensions are often fun:
my_label:
unsigned char *ptr = (unsigned char *)&&my_label;
*ptr = 5; // Will it segfault? Finding out is half the fun...
The syntax for member pointers also causes me grief, more because I don't use it often enough than because there's anything really tricky about it:
template<typename T, int T::* P>
function(T& t)
{
t.*P = 5;
}
But, really, who needs to discuss the obscure syntax in C++? With operator overloading, you can invent your own!