ichoran / kse3   0.2.6


Kerr Scala Extensions for Scala 3

Scala versions: 3.x


This repository contains the Kerr Standard Extensions to Scala 3.

These are intended to cover everything that the standard library forgot that and is commonly used for general-purpose programming and data analysis.

Kse3 has no pretenses of being an idiomatic toolchain. It is designed for high-productivity programming for people who like direct style, good error handling, and care about performance. There is no particular attempt to take a monadic or other functional approach, except inasmuch as it helps productivity. When there is a tradeoff between enabling good user code and writing "good" library code (DRY, etc.), Kse3 favors the user. Kse is supposed to take care of any necessary ugly stuff so you don't have to.

Warning: kse3 only works on Scala 3.4 and later due to its use of scala.util.boundary and extensions. It also assumes at least Java 17.

Warning: when you use extensions with Scala 3 in multiple libraries, they clobber each other because they all share the same namespace. Use import scala.language.experimental.relaxedExtensionImports everywhere!

How do I get it?

Only kse3-basics, kse3-flow, kse3-maths and some of kse3-eio (and kse3-testing) are available presently. In mill, make sure your module has a sufficiently recent 3.4 nightly, e.g.

def scalaVersion = "3.4.0-RC1-bin-20231114-18ada51-NIGHTLY"

And add at least the first line out of


to try it out. If you use some other build system, you can probably figure out from the above what you need.

Then in your code,

import kse.basics.{given, _}
import kse.flow.{given, _}
import kse.maths.{given, _}
import kse.maths.packed.{given, _}
import kse.eio.{given, _}

and you're ready to go.

(Note: I don't test without the given imports. They may work, as Kse3 generally puts givens in places where they'll be automatically found. No givens would be defined that aren't essential for functionality, so if they're there, you want to import them.)

Is this stable? Binary compatible?

Not yet and no. Eventually it will be stable. Binary compatibility is not likely to ever be an explicit goal, because Kse3 makes very heavy use of inlining. Because you have to recompile all the time anyway, there is no reason to try to maintain binary compatibility.

How do I pronounce it?

I don't hear words when I read them, so I don't have any strong opinion. I think the most conventional way to pronounce something with that spelling would be K-see, so let's go with that?


The structure of packages is still being determined. In general, they will be kse. something--a name collision with the original Kse for Scala 2, but you shouldn't use that with Scala 3 because Kse is actually still on Scala 2.12. So it's all good.


The basics module has no dependencies itself. In mill, add the dependency


and in your code,

import kse.basics.{given, _}

You have access to full-speed inlined pipe (to transform values) and tap (to act on them but pass forward the original):

val m = (1 + 2*3).pipe(x => x*x)
val n = (1 + 2*3).tap(println) + 1 // prints: 7
println(n)                         // prints: 8
println(m)                         // prints: 49

Unlike the equivalents in the Scala standard library, these are inlined and thus can be used without a loss in speed.

There are standard mutable containers for primitive and object types for counting, callbacks, and other such use:

def nextEven(m: Mu[Int]): m.type =
  m.zap(i => if i % 2 == 0 then i + 2 else i + 1)

val count = Mu(1)
for (i <- 1 to 5) nextEven(count)
println(count.value)   // prints: 10

def tokenCount(s: String, tokens: Mu[Array[String]] Or Unit): Int =
  if s.isEmpty then 0
    val tok = s.split("\\s+")

val tok = Mu(Array.empty[String])
println(tokenCount("minnow salmon bass eel", tok))  // prints: 4
println(tok.value.mkString)                         // prints: minnowsalmonbasseel

Plus there are handy methods provided on tuples, wrappers to suppress printing or use identity hash codes, and a bunch of methods that add basic ranged functionality to arrays with full hand-rolled speed by virtue of extensive use of inlines.

val a = Array(1, 2, 3, 4, 5)
val b = a.select(_ % 2 == 1)  // Array(1, 3, 5)
b(1 to End) = a  // b is now Array(1, 1, 2)
b(End - 1) = 4   // b is now Arary(1, 4, 2)
b.inject(a, 2)() // a is now Array(1, 2, 1, 4, 2)
var n = 0
a.peek()(n += _)
println(n)      // Prints 10

There is also a universal ultra-lightweight type-tagging system using string constants to refine types like String \ "name", and if you have a tuple which is entirely labelled, you can refer to the values by name by using ~

val person = ("John" \ "first", "Smith" \ "last")
val nosrep = ("Smith" \ "last", "John" \ "first")
println(person == nosrep)  // false
println(person ~ "first" == nosrep ~ "first")  // true
println(person ~ "first" == nosrep ~ "first")  // true
println(person ~ "last" == nosrep ~ "last")    // true

def welcome(who: (String \ "first", String \ "last")): Unit =
  println(s"Hello, ${who ~ "first"}")

welcome(person)          // Prints Hello, John
welcome(nosrep)          // Compile-time error; order matters
welcome(("Jane", "Doe")) // Compile-time error; labels are needed
welcome(("Jane", "Doe").label)  // Works, .label means infer labels

Use it whenever identity is really important, but types aren't specific enough!

See the test suite, or package Scaladoc, for more examples of what you could do with kse.basics!


The flow module depends only on kse.basics. In mill, add the dependency


and in your code,

import kse.flow.{given, _}

Then you have access to the left-biased unboxed sum type Or:

def little(i: Int): Int Or String =
  if i > -100 && i < 100 then Is(i)
  else Alt(s"$i is not little")

val x = little(12)
val y = little(999)

println(x)             // prints: 10
println(y)             // prints: Alt(999 is not little)
y.foreachAlt(println)  // prints: 999 is not little
x.foreachAlt(println)  // does not print anything

val z = x.map(_ * -3)
val w = z.discard{
  case i if i < 0 => "No negatives please"

println(z)             // prints: -30
println(w)             // prints: Alt("No negatives please")

You have Rust-style .? early-exit error handling for Or (and Option and Either and Try, but once you have Or, why ever use those?), and safe{ } to intercept exceptions and pack the result into A Or Throwable.

Note that .? does not need to exit an entire method; it will instead exit an Or.Ret: block (there are other variants available, but this is the most general). To exit a method, wrap the entire method body in an Or.Ret: block.

def reverseParse(s: String): Int Or String =
    safe{ s.reverse.toInt }.mapAlt(_ => s"$s is not a number in reverse")

def favorite(i: Int): Int Or String = Or.Ret:
  val x = little(i).?  // return bad case early, keep going with good value
  x * reverseParse(x.toString).?

val a = favorite(12)
val b = favorite(999)
val c = favorite(-3)
println(a)   // prints: 252
println(b)   // prints: Alt(999 is not little)
println(c)   // prints: Alt(-3 is not a number in reverse)

If you don't care about what the bad values are (usually you should!), you also have .! that can be used inside a single or chained attempt block, but otherwise works like .?:

val opt = Option(42)
val number =
    val a = favorite(999).!  // fails here
    (a < 30).!               // this would fail too
    2 * a
    val a = favorite(12).!   // succeeds, giving 21
    val b = opt.!            // gets 42 from option
    (a >= b).!               // test passes
    a * b
    -1                       // If everything failed, would give this

.! can be used on an Or, Option, Either, Try, an empty iterator (if non-empty, it will get the next item), or a Boolean test which is false. It will return the success branch if available, or will discard the error value and proceed to the next attempt or default block if it's When a Boolean test succeeds, you can chain && afterwards to run the next computation. If you want attempt to catch exceptions too, use attempt.safe and chain with .safe instead of .attempt.

There are inlined index-based loops, as fast as while but without risk of an indexing error:

val list = List("salmon", "herring", "perch")
iFor(list.iterator){ (i, s) => println(s*i) }  // prints newline, then "herring", then "perchperch"

And a variety of other nice things that you can find by perusing the ScalaDoc, the unit tests, or the code.


This exists but presently is undocumented. Feel free to look through the unit tests to see what can be done, however!


This also partly exists but is also undocumented. There are some tests, though!


This enables a very succinct but delightfully powerful testing framework, where your tests look like

T ~ myTestFunction ==== myExpectedValue --: typed[MyExpectedType]

For one-liner tests, it's incredibly effective. All the tests for kse use kse.testutilities (the package name is longer because it's nice to reserve testing for other use).

Naming Conventions

Although types and interfaces provide a lot of information about a program, names also can transmit important information about expectations for behavior (or about types or interfaces). In order to take advantage of this extra channel to convey information, Kse3 attempts to maintain the following naming conventions; violations without good reason can be considered bugs.

General principles.

  1. Readability first. If following another principle makes something hard to read, don't follow it.

  2. Convenience over documentation. Method names should be short and to the point if they are expected to be used heavily--in that case we can assume the reader knows what they mean. Only rarely-used or discouraged method names should be long. If you want to know what 'py' does (the first time), read the documentation or the source code! However, it is important to avoid method names that are so terse as to be easily confused (see: readability first).

  3. Assume infix. Even if it's not marked, assume anything could be used infix if it might possibly read more clearly. That is: infix notation is not an exception to the first principle.

Construction of objects.

  1. The default creation method is apply on a companion object, to have uniform syntax with constructors (). An apply may fail, or may correct illegal bounds and so on, but if the usual creation method fails so often that it should have an Option or Or type instead, the creation method should not be called apply.

  2. An opaque type or a class that ought usually to have its input checked but could possibly be used directly (e.g. on the underlying type) is created with the wrap method. If there is a wrap method to create it, the should be a corresponding unwrap method to get the same type back out. A wrap/unwrap/wrap cycle should produce functionally identical objects.

  3. An opaque type or a class that is just a label or adds functionality, and works on every wrapped value, will reveal its contents with underlying, if it is not container-like, or value, if it is container-like, instead of unwrap.

  4. TODO: figure out when to use of, from, parse, and so on, but these are allowed to return Option/Or types when appropriate.

Access to contents of objects.

  1. If an object contains a single unique value that is always available save in an error state (e.g. being "closed"), that value is called value.

  2. If an object may contain a single unique value, or a value correspoding to some parameters, you get that value unsafely by calling get (a new exception will be thrown on failure). If there is a stored error to propagate, grab instead of get will try to propagate the error as a catchable exception, without a new stack trace, if possible. Safe alternatives are named getSping, where Sping specifies an alterative strategy. For instance, getOrElse(bippy) should do bippy when the value is not there (perhaps bippy is a default value).

  3. If an object may be able to give values when supplied a parameter, the method used is apply. It may return an Option/Or type if errors are routine.

  4. If there is a way to return an Or Err type instead of failing on a missing single value, then ask will give the value or an Err.

  5. To access a value, if present, returning the original object, use use(f) where f acts on the contents and returns Unit. Generally a parameterized use is not encouraged, e.g. use(3)(println): use tap instead.

  6. To act on and update a modifiable value, if present, returning the original object, use zap(f) where f maps between the same types. Again, parameterized zap is not encouraged, e.g. no zap(3)(_.toUpperCase).

  7. If there is a unique modifiable value, set will set it.

Accessors and Modification

  1. Accessors to fields and field-like quantities should just be the name of the thing, e.g. foo, even if error is routine so an Or type is returned.

  2. Risky accessors, if not the normal alternative, should be named fooGet or fooGrab.

  3. An error-wrapping accessor as an alternative to a dangerous default should be called fooAsk.

  4. To re-create the same object with a different value of a field, use fooTo and the new value. If it is possible to create an object of different type, fooTo should allow it.

  5. To re-create the same object with a different value computed from the existing one, use fooOp and a function to compute the new value.

  6. To set a mutable value where setting cannot go wrong, use .foo = (i.e. the foo_= method). If it can go wrong, .fooSet is allowed to return an error value.

  7. To set or alter a mutable field inline, use .fooZap(f); this will return the object that you've modified, so it should only be provided if errors are rare.

  8. If there are both self-mutating and new-object-creating variants of the same method, the last one should have Me appended, e.g. reverseMe(). The Me notation is generally considered good form regardless for self-modifying methods, unless the naming is clunky.

Exceptions and Error Handling

  1. Anything called safe, safely, etc., will catch and package exceptions. Control flow will not be caught.

  2. Anything called nice, nicely, etc., will catch exceptions and put them and other errors into an Or Err preserving the Or Err status if it's already there. nice{ foo } || bar.nicely(f) is a sensible fallback pattern.

  3. Do not ever use Try. It catches control flow. You need to understand threading, lazy evaluation, and other situations where control flow might escape. The result will be wrong even if you catch control flow with Try.

  4. If you must catch control flow-style errors, use threadsafe{ ... }. ControlThrowable will be caught too.

More to come

There is more to come, obviously.