I'm using the mach.int library in a specification (see also this question), and I'm trying to define a constant that is of type int32:
let constant_out1: int32 = 1 in
…
However, when analyzing this, why3 returns the message:
This term has type int, but is expected to have type int32
I noticed that Bounded_int (which Int32 instantiates with type int32) has the following in it:
val of_int (n:int) : t
requires { "expl:integer overflow" in_bounds n }
ensures { to_int result = n }
However, I do not seem to be able to use this to cast 1 to int32. For example, if I use:
let constant_out1: int32 = Int32.of_int(1) in
…
I get the message unbound symbol 'Int32.of_int'. I've tried many permutations of this, all without any success. Can anyone provided guidance for how to tell why3 that I want 1 to be of type int32?
Related
I want to explore the feature of if-constexpr and try to figure out type information at compile-time.
For this purpose, I write the following code.
I expect that printTypeInfo function will return 4 for x and 3.24 for y. However, it gives me 3.1 and 3.24.
I don't know where goes wrong.
I wish decltype will infer the type to int, but it seems infer to double.
When I replace decltype with following code, it works.
enter image description here
decltype(value) in your example will always be const T&, since that's your parameter type. References aren't integral, so you always add 0.1. You could use std::remove_reference_t to get the underlying type:
auto printTypeInfo(const auto& value) {
if constexpr (std::is_integral_v<std::remove_reference_t<decltype(value)>>) {
return v + 1;
} else {
return v + 0.1;
}
}
I am trying to solve the following question on LeetCode; Write a function that takes an unsigned integer and returns the number of '1' bits it has. Constraints: The input must be a binary string of length 32.
I have written the following code for that which works fine for inputs 00000000000000000000000000001011 and 00000000000000000000000010000000 (provided internally by the website) but give output 0 for input 11111111111111111111111111111101 and in my local compiler for the last input it says "out of range"
class Solution {
// you need treat n as an unsigned value
fun hammingWeight(n:Int):Int {
var num = n
var setCountBit = 0
while (num > 0) {
setCountBit++
num= num and num-1
}
return setCountBit
}
}
To correctly convert binary string to Int and avoid "out of range error", you need to do the following (I believe LeetCode does the same under the hood):
fun binaryStringToInt(s: String): Int = s.toUInt(radix = 2).toInt()
"11111111111111111111111111111101" is equivalent to 4294967293. This is greater than Int.MAX_VALUE, so it will be represented as negative number after .toInt() convertion (-3 in this case).
Actually, this problem could be solved with one-liner in Kotlin 1.4:
fun hammingWeight(n: Int): Int = n.countOneBits()
But LeetCode uses Kotlin 1.3.10, so you need to adjust your solution to handle negative Ints as well.
Please change the type of your input variable from Int to a type like Double .At the moment The given value is bigger than the maximum value that a type Int number can store.
The Perl 6 Web site on functions says
Coercion types can help you to have a specific type inside a routine, but accept wider input. When the routine is called, the argument is automatically converted to the narrower type.
sub double(Int(Cool) $x) {
2 * $x
}
say double '21'; # 42
say double Any; # Type check failed in binding $x; expected 'Cool' but got 'Any'
Here the Int is the target type to which the argument will be coerced, and Cool is the type that the routine accepts as input.
But what is the point for the sub? Isn't $x just an Int? Why would you restrict the caller to implement Cool for the argument?
I'm doubly confused by the example because Int already is Cool. So I did an example where the types don't share a hierarchy:
class Foo { method foomethod { say 'foomethod' } }
class Bar {}
class Quux is Foo {
# class Quux { # compile error
method Bar { Bar.new }
}
sub foo(Bar(Foo) $c) {
say $c.WHAT; # (Bar)
# $c.foomethod # fails if uncommented: Method 'foomethod' not found for invocant of class 'Bar'
}
foo(Quux.new)
Here the invocant of foo is restricted to provide a Foo that can be converted to a Bar but foo cannot even call a method of Foo on $c because its type is Bar. So why would foo care that the to-be-coerced type is a Foo in the first place?
Could someone shed some light on this? Links to appropriate documentation and parts of the spec are appreciated as well. I couldn't find anything useful there.
Update Having reviewed this answer today I've concluded I had completely misunderstood what #musiKk was getting at. This was revealed most clearly in #darch's question and #musiKk's response:
#darch: Or is your question why one might prefer Int(Cool) over Int(Any)? If that's the case, that would be the question to ask.
#musiKk: That is exactly my question. :)
Reviewing the many other answers I see none have addressed it the way I now think it warrants addressing.
I might be wrong of course so what I've decided to do is leave the original question as is, in particular leaving the title as is, and leave this answer as it was, and instead write a new answer addressing #darch's reformulation.
Specify parameter type, with no coercion: Int $x
We could declare:
sub double (Int $x) { ... } # Accept only Int. (No coercion.)
Then this would work:
double(42);
But unfortunately typing 42 in response to this:
double(prompt('')); # `prompt` returns the string the user types
causes the double call to fail with Type check failed in binding $x; expected Int but got Str ("42") because 42, while looking like a number, is technically a string of type Str, and we've asked for no coercion.
Specify parameter type, with blanket coercion: Int() $x
We can introduce blanket coercion of Any value in the sub's signature:
sub double (Int(Any) $x) { ... } # Take Any value. Coerce to an Int.
Or:
sub double (Int() $x) { ... } # Same -- `Int()` coerces from Any.
Now, if you type 42 when prompted by the double(prompt('')); statement, the run-time type-check failure no longer applies and instead the run-time attempts to coerce the string to an Int. If the user types a well-formed number the code just works. If they type 123abc the coercion will fail at run-time with a nice error message:
Cannot convert string to number: trailing characters after number in '123⏏abc'
One problem with blanket coercion of Any value is that code like this:
class City { ... } # City has no Int coercion
my City $city;
double($city);
fails at run-time with the message: "Method 'Int' not found for invocant of class 'City'".
Specify parameter type, with coercion from Cool values: Int(Cool) $x
We can choose a point of balance between no coercion and blanket coercion of Any value.
The best class to coerce from is often the Cool class, because Cool values are guaranteed to either coerce nicely to other basic types or generate a nice error message:
# Accept argument of type Cool or a subclass and coerce to Int:
sub double (Int(Cool) $x) { ... }
With this definition, the following:
double(42);
double(prompt(''));
works as nicely as it can, and:
double($city);
fails with "Type check failed in binding $x; expected Cool but got City (City)" which is arguably a little better diagnostically for the programmer than "Method 'Int' not found for invocant of class 'City'".
why would foo care that the to-be-coerced type is a Foo in the first place?
Hopefully it's now obvious that the only reason it's worth limiting the coerce-from-type to Foo is because that's a type expected to successfully coerce to a Bar value (or, perhaps, fail with a friendly message).
Could someone shed some light on this? Links to appropriate documentation and parts of the spec are appreciated as well. I couldn't find anything useful there.
The document you originally quoted is pretty much all there is for enduser doc. Hopefully it makes sense now and you're all set. If not please comment and we'll go from there.
What this does is accept a value that is a subtype of Cool, and tries to transform it into an Int. At that point it is an Int no matter what it was before.
So
sub double ( Int(Cool) $n ) { $n * 2 }
can really be thought of as ( I think this is how it was actually implemented in Rakudo )
# Int is a subtype of Cool otherwise it would be Any or Mu
proto sub double ( Cool $n ) {*}
# this has the interior parts that you write
multi sub double ( Int $n ) { $n * 2 }
# this is what the compiler writes for you
multi sub double ( Cool $n ) {
# calls the other multi since it is now an Int
samewith Int($n);
}
So this accepts any of Int, Str, Rat, FatRat, Num, Array, Hash, etc. and tries to convert it into an Int before calling &infix:<*> with it, and 2.
say double ' 5 '; # 25
say double 2.5; # 4
say double [0,0,0]; # 6
say double { a => 0, b => 0 }; # 4
You might restrict it to a Cool instead of Any as all Cool values are essentially required to provide a coercion to Int.
( :( Int(Any) $ ) can be shortened to just :( Int() $ ) )
The reason you might do this is that you need it to be an Int inside the sub because you are calling other code that does different things with different types.
sub example ( Int(Cool) $n ) returns Int {
other-multi( $n ) * $n;
}
multi sub other-multi ( Int $ ) { 10 }
multi sub other-multi ( Any $ ) { 1 }
say example 5; # 50
say example 4.5; # 40
In this particular case you could have written it as one of these
sub example ( Cool $n ) returns Int {
other-multi( Int($n) ) * Int($n);
}
sub example ( Cool $n ) returns Int {
my $temp = Int($n);
other-multi( $temp ) * $temp;
}
sub example ( Cool $n is copy ) returns Int {
$n = Int($n);
other-multi( $n ) * $n;
}
None of them are as clear as the one that uses the signature to coerce it for you.
Normally for such a simple function you can use one of these and it will probably do what you want.
my &double = * * 2; # WhateverCode
my &double = * × 2; # ditto
my &double = { $_ * 2 }; # bare block
my &double = { $^n * 2 }; # block with positional placeholder
my &double = -> $n { $n * 2 }; # pointy block
my &double = sub ( $n ) { $n * 2 } # anon sub
my &double = anon sub double ( $n ) { $n * 2 } # anon sub with name
my &double = &infix:<*>.assuming(*,2); # curried
my &double = &infix:<*>.assuming(2);
sub double ( $n ) { $n * 2 } # same as :( Any $n )
Am I missing something? I'm not a Perl 6 expert, but it appears the syntax allows one to specify independently both what input types are permissible and how the input will be presented to the function.
Restricting the allowable input is useful because it means the code will result in an error, rather than a silent (useless) type conversion when the function is called with a nonsensical parameter.
I don't think an example where the two types are not in a hierarchical relationship makes sense.
Per comments on the original question, a better version of #musiKk's question "What is the point of coercions like Int(Cool)?" turned out to be:
Why might one prefer Int(Cool) over Int(Any)?
A corollary, which I'll also address in this answer, is:
Why might one prefer Int(Any) over Int(Cool)?
First, a list of various related options:
sub _Int_strong (Int $) {} # Argument must be Int
sub _Int_cool (Int(Cool) $) {} # Argument must be Cool; Int invoked
sub _Int_weak (Int(Any) $) {} # Argument must be Any; Int invoked
sub _Int_weak2 (Int() $) {} # same
sub _Any (Any $) {} # Argument must be Any
sub _Any2 ( $) {} # same
sub _Mu (Mu $) {} # Weakest typing - just memory safe (Mu)
_Int_strong val; # Fails to bind if val is not an Int
_Int_cool val; # Fails to bind if val is not Cool. Int invoked.
_Int_weak val; # Fails to bind if val is not Any. Int invoked.
_Any val; # Fails to bind if val is Mu
_Mu val; # Will always bind. If val is a native value, boxes it.
Why might one prefer Int(Cool) over Int(Any)?
Because Int(Cool) is slightly stronger typing. The argument must be of type Cool rather than the broader Any and:
Static analysis will reject binding code written to pass an argument that isn't Cool to a routine whose corresponding parameter has the type constraint Int(Cool). If static analysis shows there is no other routine candidate able to accept the call then the compiler will reject it at compile time. This is one of the meanings of "strong typing" explained in the last section of this answer.
If a value is Cool then it is guaranteed to have a well behaved .Int conversion method. So it will not yield a Method not found error at run-time and can be relied on to provide a good error message if it fails to produce a converted to integer value.
Why might one prefer Int(Any) over Int(Cool)?
Because Int(Any) is slightly weaker typing in that the argument can be of any regular type and P6 will just try and make it work:
.Int will be called on an argument that's passed to a routine whose corresponding parameter has the type constraint Int(...) no matter what the ... is. Provided the passed argument has an .Int method the call and subsequent conversion has a chance of succeeding.
If the .Int fails then the error message will be whatever the .Int method produces. If the argument is actually Cool then the .Int method will produce a good error message if it fails to convert to an Int. Otherwise the .Int method is presumably not a built in one and the result will be pot luck.
Why Foo(Bar) in the first place?
And what's all this about weak and strong typing?
An Int(...) constraint on a function parameter is going to result in either:
A failure to type check; or
An.Int conversion of the corresponding argument that forces it to its integer value -- or fails, leaving the corresponding parameter containing a Failure.
Using Wikipedia definitions as they were at the time of writing this answer (2019) this type checking and attempted conversion will be:
strong typing in the sense that a type constraint like Int(...) is "use of programming language types in order to both capture invariants of the code, and ensure its correctness, and definitely exclude certain classes of programming errors";
Currently weak typing in Rakudo in the sense that Rakudo does not check the ... in Int(...) at compile time even though in theory it could. That is, sub double (Int $x) {}; double Date; yields a compile time error (Calling double(Date) will never work) whereas sub double (Int(Cool) $x) {}; double Date; yields a run time error (Type check failed in binding).
type conversion;
weak typing in the sense that it's implicit type conversion in the sense that the compiler will handle the .Int coercion as part of carrying out the call;
explicit type conversion in the sense that the Int(...) constraint is explicitly directing the compiler to do the conversion as part of binding a call;
checked explicit type conversion -- P6 only does type safe conversions/coercions.
I believe the answer is as simple as you may not want to restrict the argument to Int even though you will be treating it as Int within the sub. say for some reason you want to be able to multiply an Array by a Hash, but fail if the args can't be treated as Int (i.e. is not Cool).
my #a = 1,2,3;
my %h = 'a' => 1, 'b' => 2;
say #a.Int; # 3 (List types coerced to the equivalent of .elems when treated as Int)
say %h.Int; # 2
sub m1(Int $x, Int $y) {return $x * $y}
say m1(3,2); # 6
say m1(#a,%h); # does not match
sub m2(Int(Cool) $x, Int(Cool) $y) {return $x * $y}
say m2('3',2); # 6
say m2(#a,%h); # 6
say m2('foo',2); # does not match
of course, you could also do this without the signature because the math operation will coerce the type automatically:
sub m3($x,$y) {return $x * $y}
say m3(#a,%h); # 6
however, this defers your type check to the inside of the sub, which kind of defeats the purpose of a signature and prevents you from making the sub a multi
All subtypes of Cool will be (as Cool requires them to) coerced to an Int. So if an operator or routine internal to your sub only works with Int arguments, you don't have to add an extra statement/expression converting to an Int nor does that operator/routine's code need to account for other subtypes of Cool. It enforces that the argument will be an Int inside of your sub wherever you use it.
Your example is backwards:
class Foo { method foomethod { say 'foomethod' } }
class Bar {}
class Quux is Bar {
method Foo { Foo.new }
}
sub foo(Foo(Bar) $c) {
#= converts $c of type Bar to type Foo
#= returns result of foomethod
say $c.WHAT; #-> (Foo)
$c.foomethod #-> foomethod
}
foo(Quux.new)
I am using a C library in my Objective C project. The C library offers the following function
void processData(...);
which can be used with 1, 2 or 3 parameters, where the first parameter is mandatory and can have different types int, double, long, float and the other two arguments are optional and have int and long values and can be in whatever order.
Examples of use of this function are:
int myInt = 2;
double myDouble = 1.23;
int dataQuality = 1;
long dataTimestamp= GET_NOW();
processData(myInt);
processData(myInt, dataQuality);
processData(myDouble, dataQuality, dataTimestamp);
processData(myDouble, dataTimestamp);
I need to make an Objetive C wrapper that uses DataType class to call processDatawith the correct parameters. The Data class has getters that allows to get the data type (first argument), its value and whether the second and third arguments have value and their value.
The problem is how to make this expansion? I think it must be done at compile time, and I think the only mechanism available in C to do so is macros. But I have never used them. The implementation should be something like this (the following is pseudocode, where the arguments list is evaluated at runtime, something that I guess should be replaced by macros in order to evaluate the arguments at compile time):
-(void) objetiveCProcessData: (Data) d {
argumentList = {}
switch (d.getDataType()) {
case INT_TYPE:
append(argumentList, d.getValueAsInt()); // <-- appends a value with type `int`
break;
case DOUBLE_TYPE:
append(argumentList, d.getValueAsDouble()); // <-- appends a value with type `double`
break;
...
}
if (d.hasQuality()) {
append(argumentList, d.getQuality());
}
if (d.hasTimeStamp()) {
append(argumentList, d.getTimestamp());
}
// Call to the C function with correct number and type of arguments
processData(argumentList);
}
I was trying out lambda functions and making a jump table to execute them, but I found g++ didn't recognize the type of the lambda function such that I could assign them to an array or tuple container.
One such attempt was this:
auto x = [](){};
decltype(x) fn = [](){};
decltype(x) jmpTable[] = { [](){}, [](){} };
On compilation I get these errors:
tst.cpp:53:27: error: conversion from ‘main()::<lambda()>’ to non-scalar type ‘main()::<lambda()>’ requested
tst.cpp:54:39: error: conversion from ‘main()::<lambda()>’ to non-scalar type ‘main()::<lambda()>’ requested
Hmmmm, can't convert from type A to non-scalar type A? What's that mean? o.O
I can use std::function to get this to work, but a problem with that is it doesn't seem to work with tuple:
function<void()> jmpTable[] = [](){}; // works
struct { int i; function<void()>> fn; } myTuple = {1, [](){}}; // works
tuple<int, function<void()>> stdTuple1 = {1, [](){}}; // fails
tuple<int, function<void()>> stdTuple2 = make_tuple(1, [](){}); // works
tst.cpp:43:58: error: converting to ‘std::tuple<int, std::function<void()> >’ from initializer list would use explicit constructor ‘std::tuple<_T1, _T2>::tuple(_U1&&, _U2&&) [with _U1 = int, _U2 = main()::<lambda()>, _T1 = int, _T2 = std::function<void()>]’
Constructor marked explicit? Why?
So my question is if I am doing something invalid or is this version just not quite up to the task?
Hmmmm, can't convert from type A to non-scalar type A? What's that mean? o.O
No, that's not a conversion to the same type. Despite having identical bodies, the different lambdas have different types. Newer versions of GCC make this clearer, and give the error message:
error: conversion from '__lambda1' to non-scalar type '__lambda0' requested
clang does even better:
error: no viable conversion from '<lambda at test.cc:2:18>' to 'decltype(x)' (aka '<lambda at test.cc:1:10>')
I can use std::function to get this to work, but a problem with that is it doesn't seem to work with tuple:
It does (with 4.5.4, at least, I don't have 4.5.3 to test), but your initialisation isn't quite right.
tuple<int, function<void()>> stdTuple1 {1, [](){}}; // lose the = to initialise stdTuple1 directly
I'm not sure about the state of n3043 in 4.5.3, but you should be able to use function pointer conversion. If I'm not misunderstanding your usage intentions, this may work for you;
void (*x)();
decltype(x) fn = [](){};
decltype(x) jmpTable[] = { [](){}, [](){} };