I want to be able to parse a string to Decl.Def but the code doesn't compile:
import scala.meta._
val s:String = ... known only at runtime
s.parse[Decl.Def].get
Error:(39, 52) don't know how to parse into scala.meta.Decl.Def
What do I need to do to parse it? Using scalameta 2.0.0
Ok, managed to find the solution:
s.parse[Stat]
Related
I am writing a web services project using http4s and everytime I write a new data object which is sent in or out of the web service, I need to write the following code
import argonaut.{Argonaut, CodecJson}
import org.http4s.{EntityDecoder, EntityEncoder}
import org.http4s.argonaut._
final case class Name (name: String, age : Int)
object Name {
implicit val codec : CodecJson[Name] =
Argonaut.casecodec2(Name.apply, Name.unapply)("name", "age")
implicit val decoder : EntityDecoder[Name] = jsonOf[Name]
implicit val encoder : EntityEncoder[Name] = jsonEncoderOf[Name]
}
Based on the number of fields in the case class, I needed to use corresponding casecodeX method (where x is the number of fields) and then pass it a list of fields.
Can you please tell me what is the best way so that I don't have to write the code which is currently in the companion object.
An idea which I have is that I should write a macro which parses the code of the Name class and then spits out the class containing the codec, encoder, decoder. But I have no idea how to go forward with the implementation of this macro.
Is there a better way?
For the codec, you can use argonaut-shapeless, specifically JsonCodec. For the encoder/decoder, you can pass jsonOf as decoder to the functions you're calling, and implicit derivation should do the rest for you. Sadly you can't get around jsonOf, it has been tried.
Also read: http://http4s.org/docs/0.15/json.html
Not really sure if it would be really better or not, but you could start with generic implicits for encoder and decoder:
implicit def decoder[A](implicit cj: CodecJson[A]): EntityDecoder[A] = jsonOf[A]
implicit val encoder[A](implicit cj: CodecJson[A]) : EntityEncoder[A] = jsonEncoderOf[A]
On that step you are getting read of 2/3 of boilerplate.
The other part is trickier: you could go with macro or reflection.
I know nothing about macro, but with reflection the reduction wouldn't be as significant to make you want to use it:
def generateCodecJson[A](implicit ClassTag[A]): CodecJson[A] = …
and you still have to provide the companion object and call that function to generate CodecJson. Not really sure if it worth effort.
I'm not familiar with Scala. But I think this situation you faced is similar in Java. In Java, all those code are imported by IDE when you inputed a token which is unknown in current namespace. You can just try and use a better IDE, such as Intellij IDEA.
I have written a parser which transforms a String to a Seq[String] following some rules. This will be used in a library.
I am trying to transform this Seq[String] to a case class. The case class would be provided by the user (so there is no way to guess what it will be).
I have thought to shapeless library because it seems to implement the good features and it seems mature, but I have no idea to how to proceed.
I have found this question with an interesting answer but I don't find how to transform it for my needs. Indeed, in the answer there is only one type to parse (String), and the library iterates inside the String itself. It probably requires a deep change in the way things are done, and I have no clue how.
Moreover, if possible, I want to make this process as easy as possible for the user of my library. So, if possible, unlike the answer in link above, the HList type would be guess from the case class itself (however according to my search, it seems the compiler needs this information).
I am a bit new to the type system and all these beautiful things, if anyone is able to give me an advice on how to do, I would be very happy!
Kind Regards
--- EDIT ---
As ziggystar requested, here is some possible of the needed signature:
//Let's say we are just parsing a CSV.
#onUserSide
case class UserClass(i:Int, j:Int, s:String)
val list = Seq("1,2,toto", "3,4,titi")
// User transforms his case class to a function with something like:
val f = UserClass.curried
// The function created in 1/ is injected in the parser
val parser = new Parser(f)
// The Strings to convert to case classes are provided as an argument to the parse() method.
val finalResult:Seq[UserClass] = parser.parse(list)
// The transfomation is done in two steps inside the parse() method:
// 1/ first we have: val list = Seq("1,2,toto", "3,4,titi")
// 2/ then we have a call to internalParserImplementedSomewhereElse(list)
// val parseResult is now equal to Seq(Seq("1", "2", "toto"), Seq("3","4", "titi"))
// 3/ finally Shapeless do its magick trick and we have Seq(UserClass(1,2,"toto"), UserClass(3,4,"titi))
#insideTheLibrary
class Parser[A](function:A) {
//The internal parser takes each String provided through argument of the method and transforms each String to a Seq[String]. So the Seq[String] provided is changed to Seq[Seq[String]].
private def internalParserImplementedSomewhereElse(l:Seq[String]): Seq[Seq[String]] = {
...
}
/*
* Class A and B are both related to the case class provided by the user:
* - A is the type of the case class as a function,
* - B is the type of the original case class (can be guessed from type A).
*/
private def convert2CaseClass[B](list:Seq[String]): B {
//do something with Shapeless
//I don't know what to put inside ???
}
def parse(l:Seq[String]){
val parseResult:Seq[Seq[String]] = internalParserImplementedSomewhereElse(l:Seq[String])
val finalResult = result.map(convert2CaseClass)
finalResult // it is a Seq[CaseClassProvidedByUser]
}
}
Inside the library some implicit would be available to convert the String to the correct type as they are guessed by Shapeless (similar to the answered proposed in the link above). Like string.toInt, string.ToDouble, and so on...
May be there are other way to design it. It's just what I have in mind after playing with Shapeless few hours.
This uses a very simple library called product-collecions
import com.github.marklister.collections.io._
case class UserClass(i:Int, j:Int, s:String)
val csv = Seq("1,2,toto", "3,4,titi").mkString("\n")
csv: String =
1,2,toto
3,4,titi
CsvParser(UserClass).parse(new java.io.StringReader(csv))
res28: Seq[UserClass] = List(UserClass(1,2,toto), UserClass(3,4,titi))
And to serialize the other way:
scala> res28.csvIterator.toList
res30: List[String] = List(1,2,"toto", 3,4,"titi")
product-collections is orientated towards csv and a java.io.Reader, hence the shims above.
This answer will not tell you how to do exactly what you want, but it will solve your problem. I think you're overcomplicating things.
What is it you want to do? It appears to me that you're simply looking for a way to serialize and deserialize your case classes - i.e. convert your Scala objects to a generic string format and the generic string format back to Scala objects. Your serialization step presently is something you seem to already have defined, and you're asking about how to do the deserialization.
There are a few serialization/deserialization options available for Scala. You do not have to hack away with Shapeless or Scalaz to do it yourself. Try to take a look at these solutions:
Java serialization/deserialization. The regular serialization/deserialization facilities provided by the Java environment. Requires explicit casting and gives you no control over the serialization format, but it's built in and doesn't require much work to implement.
JSON serialization: there are many libraries that provide JSON generation and parsing for Java. Take a look at play-json, spray-json and Argonaut, for example.
The Scala Pickling library is a more general library for serialization/deserialization. Out of the box it comes with some binary and some JSON format, but you can create your own formats.
Out of these solutions, at least play-json and Scala Pickling use macros to generate serializers and deserializers for you at compile time. That means that they should both be typesafe and performant.
I want to save an object (an instance of a class) to a file. I didn't find any valuable example of it. Do I need to use serialization for it?
How do I do that?
UPDATE:
Here is how I tried to do that
import scala.util.Marshal
import scala.io.Source
import scala.collection.immutable
import java.io._
object Example {
class Foo(val message: String) extends scala.Serializable
val foo = new Foo("qweqwe")
val out = new FileOutputStream("out123.txt")
out.write(Marshal.dump(foo))
out.close
}
First of all, out123.txt contains many extra data and it was in a wrong encoding. My gut tells me there should be another proper way.
On the last ScalaDays Heather introduced a new library which gives a new cool mechanism for serialization - pickling. I think it's would be an idiomatic way in scala to use serialization and just what you want.
Check out a paper on this topic, slides and talk on ScalaDays'13
It is also possible to serialize to and deserialize from JSON using Jackson.
A nice wrapper that makes it Scala friendly is Jacks
JSON has the following advantages
a simple human readable text
a rather efficient format byte wise
it can be used directly by Javascript
and even be natively stored and queried using a DB like Mongo DB
(Edit) Example Usage
Serializing to JSON:
val json = JacksMapper.writeValueAsString[MyClass](instance)
... and deserializing
val obj = JacksMapper.readValue[MyClass](json)
Take a look at Twitter Chill to handle your serialization: https://github.com/twitter/chill. It's a Scala helper for the Kyro serialization library. The documentation/example on the Github page looks to be sufficient for your needs.
Just add my answer here for the convenience of someone like me.
The pickling library, which is mentioned by #4lex1v, only supports Scala 2.10/2.11 but I'm using Scala 2.12. So I'm not able to use it in my project.
And then I find out BooPickle. It supports Scala 2.11 as well as 2.12!
Here's the example:
import boopickle.Default._
val data = Seq("Hello", "World!")
val buf = Pickle.intoBytes(data)
val helloWorld = Unpickle[Seq[String]].fromBytes(buf)
More details please check here.
I am new to Scala programming and I wanted to read a properties file in Scala.
I can't find any APIs to read a property file in Scala.
Please let me know if there are any API for this or other way to read properties files in Scala.
Beside form Java API, there is a library by Typesafe called config with a good API for working with configuration files of different types.
You will have to do it in similar way you would with with Scala Map to java.util.Map. java.util.Properties extends java.util.HashTable whiche extends java.util.Dictionary.
scala.collection.JavaConverters has functions to convert to and fro from Dictionary to Scala mutable.Map:
val x = new Properties
//load from .properties file here.
import scala.collection.JavaConverters._
scala> x.asScala
res4: scala.collection.mutable.Map[String,String] = Map()
You can then use Map above. To get and retrieve. But if you wish to convert it back to Properties type (to store back etc), you might have to type cast it manually then.
You can just use the Java API.
Consider something along the lines
def getPropertyX: Option[String] = Source.fromFile(fileName)
.getLines()
.find(_.startsWith("propertyX="))
.map(_.replace("propertyX=", ""))
I am converting some of my java code to scala and I would like to be able to get a specific header and return it as a string.
In java I have:
return request().getHeader("myHeader")
I have been unable to achieve the same thing in scala. Any help would be greatly appreciated! Thanks!
You could write:
request.get("myHeader").orNull
If you wanted something essentially the same as your Java line. But you don't!
request.get("myHeader") returns an Option[String], which is Scala's way of encouraging you to write code that won't throw null pointer exceptions.
You can process the Option in various ways. For example, if you wanted to supply a default value:
val h: String = request.get("myHeader").getOrElse("")
Or if you want to do something with the header if it exists:
request.foreach { h: String => doSomething(h) }
Or just:
request foreach doSomething
See this cheat sheet for more possibilities.
Accepted answer doesn't work for scala with playframework 2.2:
request.get("myHeader").getOrElse("")
It gives me the below error:
value get is not a member of
play.api.mvc.Request[play.api.mvc.AnyContent]
use below
request.headers.get("myHeader").getOrElse("")