Starting with the helloworld, I am trying to learn me some pony by making an app which echoes back to the user what they have typed in. This works, except for the one line where I _env.out.print(_buffer).
class MyInput iso is StdinNotify
var _env: Env
var _buffer: Array[U8]
new iso create(env: Env) =>
_env = env
_buffer = Array[U8].create()
fun ref apply(data: Array[U8] iso) =>
for item in (consume data).values() do
if item == 0x0A then
_env.out.print(_buffer)
_buffer = Array[U8].create()
end
_buffer.push(item)
end
The compile errors I get are:
argument not a subtype of parameter
_env.out.print(_buffer)
^
parameter type: ByteSeq val
be print(data: ByteSeq) =>
^
argument type: Array[U8 val] ref
_env.out.print(_buffer)
From reading the source in github I think that Array[U8] should qualify as a ByteSeq, so I guess that it's a problem with the capabilities. I've tried a few things but I can't seem to copy my ref Array into a val Array. The closest I got was let out: Array[U8] box = _buffer
thanks to the pony mailing list and more trial and error I've got it to work. (thanks sean allen)
actor Main
new create(env: Env) =>
env.out.print("welcome to the echo chamber")
var input = recover iso MyInput.create(env) end
env.input(consume input)
class MyInput is StdinNotify
var _env: Env
var _buffer: Array[U8] iso
new create(env: Env) =>
_env = env
_buffer = recover iso Array[U8].create() end
fun ref apply(data: Array[U8] iso) =>
for item in (consume data).values() do
if item == 0x0A then
let bufferOutIso = _buffer = recover iso Array[U8].create() end
_env.out.print(consume bufferOutIso)
else
_buffer.push(item)
end
end
so in case any of this needs explaining,
it turns out that although print asks for a val argument, it will take an iso, for reasons I don't fully understand
when using iso, you need to understand recover blocks and destructive read
I made a mistake putting iso in the class definition, I only needed to create the instance in a recover iso block
Related
I'm working on a command line tool written in Scala which is executed as:
sbt "run --customerAccount 1234567"
Now, I wish to make this flexible to accept "--CUSTOMERACCOUNT" or --cUsToMerAccount or --customerACCOUNT ...you get the drift
Here's what the code looks like:
lazy val OptionsParser: OptionParser[Args] = new scopt.OptionParser[Args]("scopt") {
head(
"XML Generator",
"Creates XML for testing"
)
help("help").text(s"Prints this usage message. $envUsage")
opt[String]('c', "customerAccount")
.text("Required: Please provide customer account number as -c 12334 or --customerAccount 12334")
.required()
.action { (cust, args) =>
assert(cust.nonEmpty, "cust is REQUIRED!!")
args.copy(cust = cust)
}
}
I assume the opt[String]('c', "customerAccount") does the pattern matching from the command line and will match with "customerAccount" - how do I get this to match with "--CUSTOMERACCOUNT" or --cUsToMerAccount or --customerACCOUNT? What exactly does the args.copy (cust = cust) do?
I apologize if the questions seem too basic. I'm incredibly new to Scala, have worked in Java and Python earlier so sometimes I find the syntax a little hard to understand as well.
You'd normally be parsing the args with code like:
OptionsParser.parse(args, Args())
So if you want case-insensitivity, probably the easiest way is to canonicalize the case of args with something like
val canonicalized = args.map(_.toLowerCase)
OptionsParser.parse(canonicalized, Args())
Or, if you for instance wanted to only canonicalize args starting with -- and before a bare --:
val canonicalized =
args.foldLeft(false -> List.empty[String]) { (state, arg) =>
val (afterDashes, result) = state
if (afterDashes) true -> (arg :: result) // pass through unchanged
else {
if (arg == "==") true -> (arg :: result) // move to afterDash state & pass through
else {
if (arg.startsWith("--")) false -> (arg.toLowerCase :: result)
else false -> (arg :: result) // pass through unchanged
}
}
}
._2 // Extract the result
.reverse // Reverse it back into the original order (if building up a sequence, your first choice should be to build a list in reversed order and reverse at the end)
OptionsParser.parse(canonicalized, Args())
Re the second question, since Args is (almost certainly) a case class, it has a copy method which constructs a new object with (most likely, depending on usage) different values for its fields. So
args.copy(cust = cust)
creates a new Args object, where:
the value of the cust field in that object is the value of the cust variable in that block (this is basically a somewhat clever hack that works with named method arguments)
every other field's value is taken from args
I have following custom hook
function useConstant(fn) {
const ref = React.useRef()
if (!ref.current) {
ref.current = fn()
}
return ref.current
}
and it seems quite hard to port this to reasonml, I have to use type cast twice, what's the ideal way?
external toAny: 'a => 'b = "%identity";
external toBool: 'a => bool = "%identity";
let useConstant = (fn: unit => 'a) => {
let ref: React.Ref.t('a) = toAny(React.useRef());
if (!toBool(React.Ref.current(ref))) {
React.Ref.setCurrent(ref, fn());
};
React.Ref.current(ref);
};
If I understand the purpose of the hook correctly, it's really just a reimplementation of React.useMemo. But for the sake of learning, here's an implementation that should work.
let useLazy = (fn: unit => 'a): 'a => {
let ref = React.useRef(None);
switch (React.Ref.current(ref)) {
| Some(value) => value
| None =>
let value = fn();
React.Ref.setCurrent(ref, Some(value));
value;
};
};
It uses the option type, which is specifically designed for cases like this. If there's no value, we represent that using options None value, and if there is a value we use Some. Instead of using if with JavaScript's semantically unclear concept of truthiness, we pattern match on the option using switch to find that it's None and the value needs to be computed, or Some to get at the value.
The use of option and pattern matching is really common in Reason code, so it's one you should really try to understand using the links provided above for more details if needed.
Note that you could also have used Lazy for this. But that's far less commonly used and therefore also much less useful to learn.
I am using this fork of LMDBjni:https://github.com/deephacks/lmdbjni to
form the backend of a medium-sized databases project in scala.
I've been hitting a EXCEPTION_ACCESS_VIOLATION (0xc0000005) in the JNI code for this LMDB, and would like to know whether there is anything obvious I'm doing wrong or what the next steps for debugging should be. I'm not exactly sure what I'm looking for, so I'm going to list as much information about what's happening as I can and hope the symptoms make sense to somebody.
The access violation occurs in a Database with a single key mapping to a single 8-byte value, on approximately the 4000th access to that database (the exact number appears to be the same with each run), suggesting that this is a deterministic problem.
I believe I only have one thread accessing the database at a time, and regardless, in my understanding, as the operation is wrapped in a transaction, concurrent accesses should not matter anyway.
By looking through stack traces and printing values the issue comes from this generic construction I wrote for building transactions.
My code that causes the issue is here, the crash occurs in the marked db.get() call:
def transactionalGetAndSet[A](
key: Key,
db: Database
)(
compute: A => LMDBEither[A]
)(
implicit sa: Storeable[A],
env: Env
): LMDBEither[A] = {
import org.fusesource.lmdbjni.Transaction
// get a new transaction
val tx: Transaction = instance.env.createWriteTransaction()
println("tx = " + tx + " id = " + tx.getId)
// get the key as an Array[Byte]. This is done by converting the key to a base64 string then converting that to bytes (so arbitary objects can be made into keys)
val k = key.render
println("Key = " + key + " Rendered = " + new String(k))
// instantiate a result value, so there is something if it fails
var res: LMDBEither[A] = NoResult.left // initialise the result as a failure to begin with
try {
res = for { // This for construction chains together operations that return LMDBEithers into one LMDBEither
bytes <- LMDBEither(db.get(tx, k)) // error occurs in this Database.get() call
_ = println("bytes = " + bytes)
a <- sa.fromBytes(safeRetrieve(bytes)) // sa is effectively an unmarshaller/unmarshaller object which converts Vector[Byte] => LMDBEither[A]
_ = println("a = " + a)
res <- compute(a) // get the next value for the value at the key
_ = println("res = " + res)
_ <- LMDBEither(db.put(tx, k, sa.toBytes(res).toArray))
} yield a // effectively, if all these steps worked, res == Right(a)
res // return the result
} finally {
// Make sure you either commit or rollback to avoid resource leaks.
if (res.isRight) tx.commit() // if the result is not an error (ie Either.isRight is true)
else tx.abort()
tx.close()
}
}
Where LMDBEither[A] is an alias for Either[E, A] for a specific error type E, and LMDBEither(x) is a function that lifts an expression that might throw exceptions during execution into an LMDBEither, catching any exceptions.
the function safeRetrieve converts a possibly null Array[Byte] into a definitely not null Vector[Byte], as follows:
private def safeRetrieve(bytes: Array[Byte]): Vector[Byte] =
Option(bytes).fold(Vector[Byte]()){ // if the array is null, convert to an empty vector, otherwise call the array's wrapper's vector
arr =>
println("Vector = " + arr.toVector)
arr.toVector
}
To the best of my knowledge, this does not modify the memory where the array is stored (LMDB's protected memory)
The values printed up to and including the crash are as follows:
tx = org.fusesource.lmdbjni.Transaction#391cec1f id = 15104
Key = Vector(Objects) Rendered = 84507411390877848991196161
#
# A fatal error has been detected by the Java Runtime Environment:
#
# EXCEPTION_ACCESS_VIOLATION (0xc0000005) at pc=0x000000018002453f, pid=10220, tid=7268
#
# JRE version: Java(TM) SE Runtime Environment (8.0_60-b27) (build 1.8.0_60-b27)
# Java VM: Java HotSpot(TM) 64-Bit Server VM (25.60-b23 mixed mode windows-amd64 compressed oops)
# Problematic frame:
# C [lmdbjni-64-0-7710432736670562378.4+0x2453f]
#
# Failed to write core dump. Minidumps are not enabled by default on client versions of Windows
#
# An error report file with more information is saved as:
# C:\dev\PartIIProject\hs_err_pid10220.log
#
# If you would like to submit a bug report, please visit:
# http://bugreport.java.com/bugreport/crash.jsp
# The crash happened outside the Java Virtual Machine in native code.
# See problematic frame for where to report the bug.
#
Which is more evidence that the mdb_get() fails.
the full contents of the file referenced are here: https://pastebin.com/v6AmFBjq
Again, I would be extremely grateful for the small chance that anyone could point me in the right direction. What next steps should I be taking?
Can someone please explain whats the difference between the two in Entity Framework.
Example1:
obj = new TicketsEntities();
var depts = obj.DEPARTMENTs.Select( x => x);
string str = depts.GetType().ToString();
In this case str prints --- System.Data.Entity.Infrastructure.DbQuery`1[LINQu.Models.DEPARTMENT]
Example2:
obj = new TicketsEntities();
var depts = obj.DEPARTMENTs;
string str = depts.GetType().ToString();
In this case str prints --- System.Data.Entity.DbSet`1[LINQu.Models.DEPARTMENT]
In either case when we loop through the depts we get same result , so what is the difference between the two , and which one is preferred ?
The DbSet represents the set of data and can manipulate the data by exposing methods like Add, Update, Remove.
The DbQuery represents a Linq query that is executed on a set of data. It does not have the Add, Update and Remove methods.
In your case I think there is no real difference, but for simplicity sake I would pick your second example since the Select(x=>x) is not neccessary.
I have this class in which i try to initialize array attributes with query results:
class data
minute: []
hour: []
constructor: () ->
findMin = events.find({"aggr":"minute"}).sort({$natural:-1}).limit(120)
findHour = events.find({"aggr":"hour"}).sort({$natural:-1}).limit(14)
findMin.execFind (errMin, resMin) ->
for recMin in resMin
#minute.push recMin
findHour.execFind (errH, resH) ->
for recH in resH
#hour.push recH
So i call smth = new data() and console.log smth and get an empty attributes and an error about undefined not having 'push' method. While i can understand an error i cant get why my arrays are empty. Dont get me wrong - i know this error causes them to be empty, but i tried several kinds of variants. And ive read about acync and callbacks, but still don't have a clue how to use callbacks not to 'alert' smth, but to use it afterwards. If you could help me with that or with some links that could - i would appreciate it SO much.
You have two issues. The one that is causing the error you are observing is that '#' inside of your two callbacks is not bound to your data instance, so you need to use =>. Secondly, and as pointed out by #AaronDufour, your hour and minute arrays are declared at the class level so they will be shared between every instance of data, which I doubt is what you want, so you need to move them into your constructor.
class data
constructor: () ->
#minute = []
#hour = []
findMin = events.find({"aggr":"minute"}).sort({$natural:-1}).limit(120)
findHour = events.find({"aggr":"hour"}).sort({$natural:-1}).limit(14)
findMin.execFind (errMin, resMin) =>
for recMin in resMin
#minute.push recMin
findHour.execFind (errH, resH) =>
for recH in resH
#hour.push recH
I assume you want minute and hour to be instance variables? They must be initialized in the constructor. The way you're doing it now, they're on the prototype, so it won't work properly. Try this:
class data
constructor: () ->
#minute = []
#hour = []
findMin = events.find({"aggr":"minute"}).sort({$natural:-1}).limit(120)
findHour = events.find({"aggr":"hour"}).sort({$natural:-1}).limit(14)
findMin.execFind (errMin, resMin) =>
for recMin in resMin
#minute.push recMin
findHour.execFind (errH, resH) =>
for recH in resH
#hour.push recH