Haoyi's Programming Blog

Warts of the Scala Programming Language

Posted 2017-05-27

Scala is my current favorite general-purpose programming language. However, it definitely has its share of flaws. While some are deep trade-offs in the design of the language, others are trivial, silly issues which cause frustration far beyond their level of sophistication: "warts". This post will explore some of what, in my opinion, are the warts of the Scala programming language, to hopefully raise awareness of their existence as problems and build a desire to fix them in the broader community.


About the Author: Haoyi is a software engineer, an early contributor to Scala.js, and the author of many open-source Scala tools such as the Ammonite REPL and FastParse.

If you've enjoyed this blog, or enjoyed using Haoyi's other open source libraries, please chip in (or get your Company to chip in!) via Patreon so he can continue his open-source work


I think that Scala has a disproportionate ratio of superficial-warts to deep-design-problems. While many languages have a relatively clean superficial syntax and semantics that hides a crazy and unpredictable core logic, Scala in my opinion has a relatively elegant core logic that's overlaid by a gnarly, messy superficial syntax and semantics.

Not every problem with a programming language is a "wart". Many problems arise from deep design decisions and trade-offs, where there often isn't a "correct" solution, or where proposed solutions often come with their own share of (often unsolved) problems.

However, many problems are simply incidental. They exist for no particular reason, have no particular benefit for being around, and have "obvious" solutions that would be non-controversial. They really should have been fixed years ago, though the second best time to fix them starts today.

This post will cover some things that I don't consider warts, to set the stage for exploring an incomplete list of things I consider the warts of the Scala programming language

Not Warts

Just because people complain about something doesn't mean it's a "wart".

Here are a few of things in the Scala language that I don't consider to be warts:

Universal Equality

@ val x = "hello"
x: String = "hello"

@ val y = 123
y: Int = 123

@ x == y
res68: Boolean = false

Scala lets you compare any two values for equality via the == operator, which forwards to the Java .equals method under the hood defined a

def equals(other: Any): Boolean

This allows for all sorts of human mistakes to go uncaught at compile time: the above example will never return true, regardless of the values of x and y, and the compiler should be able to figure that out and tell us. While this example is short and the mistake is obvious, in larger examples it's much less obvious, e.g. in this example from Lincon Atkinson's blog:

> "foobar".toList == List('f','o','o','b','a,'r')
false

Despite the fact that this seems to be letting through an obvious class of errors, it's not clear to me what the "correct" way of fixing it is:

The unsafe-ness of equality is a problem: making mistakes like this is very common in my experience. e.g. when refactoring a code to change Strings to an Id class, you can fix all the compile errors, but all the equality checks are now invalid, and without the compiler's help you need to go hunt them all down manually. Furthermore, I'm sure there is a solution out there that can make things better, but the solution isn't "trivial" or "obvious" enough to make me consider this a Wart.

Running on the JVM

Scala has traditionally run on the JVM. It now runs on Javascript with Scala.js, and more recently on LLVM with Scala Native, but the heart of the ecosystem and all the tooling runs on the JVM.

This gives Scala all the benefits of the JVM: good stack traces, monitoring/deployment tools, a fast JIT and good garbage collectors, a tremendous ecosystem of libraries

This also gives Scala all the constraints of the JVM: boxing everywhere resulting in unnecessary GC pressure, a disk-heavy binary format for classfiles and jars, and a tremendously slow startup time for any non-trivial program.

For example, the Ammonite script-runner (which is a medium-sized command-line tool) spends up to half a second just classloading when running a script, before counting any actual program-logic being run:

JvmSlow

However, the reliance on the JVM is deep enough to be a core feature of the Scala language. It's debatable whether Scala would have been able to achieve the success it has without piggy-backing on all the JVM has to offer. While it would be nice to slowly reduce that reliance, which is happening now with Scala.js and Scala-Native, it's a hard trade-off that I can't consider a Wart.

Type-erasure

@ val x = List(1, 2, 3)
x: List[Int] = List(1, 2, 3)

@ val y = x.asInstanceOf[List[String]]
y: List[String] = List(1, 2, 3)

@ println(y.last)
java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String
  $sess.cmd76$.<init>(cmd76.sc:1)
  $sess.cmd76$.<clinit>(cmd76.sc:-1)

On the JVM, and now on the Javascript platform using Scala.js, generic types are erased: that means you can asInstanceOf or isInstanceOf or pattern-match against them, and they always are considered equal. That results in the above behavior, where you can cast a List[Int] into a List[String] without any errors (though it gives off a warning) and it only crashes at runtime when you try to extracta value from the already casted list!

This behavior is clearly unsafe, and can result in weird and hard-to-track-down problems. However, it's not clear to me what the correct behavior is:

// Scala.js
val x = List(1, 2, 3)
val y = x.asInstanceOf[List[String]]
println(y.last) // 3

"Full" erasure on Scala.js for Javascript types also allows for several interesting features, such as phantom-types and zero-overhead wrapper-types, that are difficult/impossible to do on the JVM without jumping through hoops. It also allows much more aggressive optimizations, since the optimizer no longer needs to keep adding guards to throw ClassCastExceptions if you get a cast wrong. The cost of this is you get more confusing error messages if you write invalid casts. In my experience, this is basically a non-issue in practice, since "typical" Scala programs tend not to rely on casts.

Lastly, it could be argued that having "Full" erasure helps enforce parametricity/encapsulation: you can no longer check what class something is when calling a method, and are thus forced to rely on whatever abstract interface you are provided with.

Hence, I don't know what the correct answer is here. There are good arguments why both "more erasure" and "less erasure" are good things, so without a obvious better-way I don't consider this a Wart.

Implicits

Scala allows implicit parameters, which can be passed manually but can also be automatically passed to functions that require them based on their type. For example, in the following case, we call the repeat function twice: once passing in count manually as 2, an once passing it implicitly via the implicit val c which is 3:

@ def repeat(s: String)(implicit count: Int) = s * count
defined function repeat

@ repeat("hello")(2)
res78: String = "hellohello"

@ repeat("hello")
cmd79.sc:1: could not find implicit value for parameter count: Int
val res79 = repeat("hello")
                  ^
Compilation Failed

@ implicit val c = 3
c: Int = 3

@ repeat("hello")
res80: String = "hellohellohello"

Scala also allows implicit conversions, that can be used to define automatic conversions from out type to another:

@ case class Name(s: String){
    require(s.nonEmpty)
  }
defined class Name

@ val n1 = Name("hello")
n1: Name = Name("hello")

@ val n2 = Name("")
java.lang.IllegalArgumentException: requirement failed
  scala.Predef$.require(Predef.scala:264)
  $sess.cmd81$Name.<init>(cmd81.sc:2)
  $sess.cmd83$.<init>(cmd83.sc:1)
  $sess.cmd83$.<clinit>(cmd83.sc:-1)

@ implicit def autoName(s: String): Name = Name(s)
defined function autoName

@ val n3: Name = "hello"
n3: Name = Name("hello")

@ val n3: Name = ""
java.lang.IllegalArgumentException: requirement failed
  scala.Predef$.require(Predef.scala:264)
  $sess.cmd81$Name.<init>(cmd81.sc:2)
  $sess.cmd84$.autoName(cmd84.sc:1)
  $sess.cmd86$.<init>(cmd86.sc:1)
  $sess.cmd86$.<clinit>(cmd86.sc:-1)

While this behavior may be confusing, unintuitive and hard-to-debug sometimes, it also forms the basis for a large number of common Scala design patterns. Scala wouldn't be Scala without implicits.

The actual implementation of implicits contains many strange corner cases, in addition to out-right bugs, but implicits themselves are so core to Scala that I don't think they could be considered a "wart"

Warts

Warts are somewhere on the spectrum between design issues and outright bugs. These are things that, I think, have obvious solutions that would be both easy and relatively uncontroversial to fix, that nonetheless cause annoyance and frustration disproportionate given how trivial the issue is.

The warts I'm going to discuss are:

Weak eta-expansion

Scala maintains a distinction between "functions" and "methods": in general, methods are things you call on an object, whereas functions are objects themselves. However, since they're so similar ("things you can call"), it gives you a way to easily wrap a method in a function object called "eta expansion"

@ def repeat(s: String, i: Int) = s * i
defined function repeat

@ repeat("hello", 2)
res89: String = "hellohello"

@ val func = repeat _
func: (String, Int) => String = $sess.cmd90$$$Lambda$2796/1082786554@2a3983b9

@ func("hello", 3)
res91: String = "hellohellohello"

Above, we use the underscore _ to assign repeat _ to a value func, which is then a function object we can call. This can happen automatically, without the _, based on the "expected type" of the place the method is being used. For example, if we expect func to be a (String, Int) => String, we can assign repeat to it without the _:

@ val func: (String, Int) => String = repeat
func: (String, Int) => String = $sess.cmd92$$$Lambda$2803/669946146@46226d53

@ func("hello", 3)
res92: String = "hellohellohello"

Or by stubbing out the arguments with _ individually:

@ val func = repeat(_, _)
func: (String, Int) => String = $sess.cmd98$$$Lambda$2832/1025548997@358b1f86

This works, but has a bunch of annoying limitations. Firstly, even though you can fully convert the method repeat into a (String, Int) => String value using _, you cannot partially convert it:

@ val func = repeat("hello", _)
cmd4.sc:1: missing parameter type for expanded function 
((x$1: <error>) => repeat("hello", x$1))
val func = repeat("hello", _)
                           ^
Compilation Failed

Unless you know the the "expected type" of func, in which case you can partially convert it:

@ val func: Int => String = repeat("hello", _)
func: Int => String = $sess.cmd93$$$Lambda$2808/1138545802@2c229ed2

Or you provide the type to the partially-applied-function-argument _ manually:


@ repeat("hello", _: Int) res4: Int => String = $sess.cmd4$$$Lambda$1988/1407003104@5eadc347

This is a bit strange to me. If I can easily convert the entire repeat method into a function without specifying any types, why can I not convert it into a function if I already know one of the arguments? After all, I have provided strictly more information in the repeat("hello", _) case than I have in the repeat(_, _) case, and yet somehow type inference got worse!

Furthermore, there's a more fundamental issue: if I know that repeat is a method that takes two arguments, why can't I just do this?

@ val func = repeat
cmd99.sc:1: missing argument list for method repeat in object cmd88
Unapplied methods are only converted to functions when a function type is expected.
You can make this conversion explicit by writing `repeat _` or `repeat(_,_)` instead of `repeat`.
val func = repeat
           ^
Compilation Failed

After all, since the compiler already knows that repeat is a method, and that it doesn't have it's arguments provided, why not convert it for me? Why force me to go through the _ or (_, _) dance, or why ask me to provide an expected type for func if it already knows the type of repeat?

In other languages with first-class functions, like Python, this works fine:

>>> def repeat(s, i):
...     return s * i
...

>>> func = repeat

>>> func("hello", 3)
'hellohellohello'

The lack of automatic eta-expansion results in people writing weird code to work around it, such as this example from ScalaMock:

"drawLine" should "interact with Turtle" in {
  // Create mock Turtle object
  val mockedTurtle = mock[Turtle]
 
  // Set expectations
  (mockedTurtle.setPosition _).expects(10.0, 10.0)
  (mockedTurtle.forward _).expects(5.0)
  (mockedTurtle.getPosition _).expects().returning(15.0, 10.0)
 
  // Exercise System Under Test
  drawLine(mockedTurtle, (10.0, 10.0), (15.0, 10.0))
}

Here, the weird (foo _) dance is something that they have to do purely because of this restriction in eta-expansion.

While I'm sure there are good implementation-reasons why this doesn't work, I don't see any reason this shouldn't work from a language-semantics point of view. From a user's point of view, methods and functions are just "things you call", and Scala is generally successful and not asking you to think about the distinction between them.

However, in cases like this, I think there isn't a good reason the compiler shouldn't try a bit harder to figure out what I want before giving up and asking me to pepper _s or expected types all over the place. The compiler already has all the information it needs - after all, it works if you put an _ after the method - and it just needs to use that information when the _ isn't present.

Callers of zero-parameter methods can decide how many parens to use

Scala lets you leave off empty-parens lists when calling functions. This looks kind of cute when calling getters:

@ def getFoo() = 1337
defined function foo

@ getFoo()
res8: Int = 1337

@ getFoo
res9: Int = 1337

However, it doesn't really make sense when you consider how this works in most other languages, such as Python:

>>> def getFoo():
...     return 1337
...
>>> getFoo()
1337
>>> func = getFoo
>>> func()
1337

After all, if getFoo() is a Int, why shouldn't getFoo without the parens be a () => Int? After all, calling a () => Int with parens give you an Int. However, in Scala methods are "special", as shown above, and methods with empty parens lists are treated even more specially.

Furthermore, this feature really doesn't make sense when you start pushing it:

@ def bar()()()()() = 2
defined function bar

@ bar
res11: Int = 2

@ bar()
res12: Int = 2

@ bar()()
res13: Int = 2

@ bar()()()
res14: Int = 2

@ bar()()()()
res15: Int = 2

@ bar()()()()()
res16: Int = 2

@ bar()()()()()()
cmd17.sc:1: Int does not take parameters
val res17 = bar()()()()()()
                         ^
Compilation Failed

Is this really the behavior we expect in a statically-typed language, that you can call this method with any number of argument lists 0 < n <= 5 and it'll do the same thing regardless? What on earth is the type of bar? The Scala community likes to think that it's "definition-side variance" is better than Java's "use-site variance", but here we have Scala providing definition-site parens where every caller of bar can pick and choose how many parens they want to pass.

I think the solution to this is clear: methods should be called with as many sets of parentheses as they are defined with (excluding implicits). Any method call missing parens should be eta-expanded into the appropriate function value.

Concretely, that means that given these two functions:

object thing{
  def head: T = ???
  def next(): T = ???
}

They currently behave like this:

val first1: T = thing.head   // Works!
val first2: T = thing.head() // Compile Error: T is not a function and cannot be called
val first3: T = thing.next   // Works!
val first4: T = thing.next() // Works!

And will there-after behave like this:

val first1: T = thing.head   // Works!
val first2: T = thing.head() // Compile Error: T is not a function and cannot be called
val first3: T = thing.next   // Compile Error: found () => T, expected T
val first4: T = thing.next() // Works!

Notably, this does not take away the ability to control how many empty-parens a function is called with; rather, it shifts that decision from the user of a function to the author of a function. Since the author of a function already decides everything else about it (It's name, arguments, return type, implementation, ...) giving the author the decision over empty-parens would not be unprecedented.

No-parens "property" functions would still be possible, the author of the function would just need to define it without parens, as is already possible:

@ def baz = 3
defined function baz

@ baz
res11: Int = 3

@ baz()
cmd12.sc:1: Int does not take parameters
val res12 = baz()
               ^
Compilation Failed

The only reason I've heard for this feature is to "let you call Java getFoo methods without the parens", which seems like an exceedingly weak justification for a language feature that so thoroughly breaks the expectations of a statically-typed language. If that was the problem, one option would be to allow use-site optional empty-parentheses only at Java call-sites or Scala call-sites with a particular annotation (@optionalParens def foo = ...?). This would limit the scope of this behavior to a mild Java-interop quirk (one of many), rather than a wart affecting the core of the Scala programming language

Needing curlies/case for destructuring anonymous functions

Scala lets you create anonymous functions with a x => x + 1 syntax:

@ Seq(1, 2, 3).map(x => x + 1)
res44: Seq[Int] = List(2, 3, 4)

But if you want to have the function work on e.g. Tuples, that doesn't work:

@ Seq((1, 2), (3, 4), (5, 6)).map((x, y) => x + y + 1)
cmd45.sc:1: missing parameter type
Note: The expected type requires a one-argument function accepting a 2-Tuple.
      Consider a pattern matching anonymous function, `{ case (x, y) =>  ... }`
val res45 = Seq((1, 2), (3, 4), (5, 6)).map((x, y) => x + y + 1)
                                             ^
cmd45.sc:1: missing parameter type
val res45 = Seq((1, 2), (3, 4), (5, 6)).map((x, y) => x + y + 1)
                                                ^
Compilation Failed

And you need to then swap over to a similar-but-annoyingly-different { case ... => ...} syntax:

@ Seq((1, 2), (3, 4), (5, 6)).map{case (x, y) => x + y + 1}
res45: Seq[Int] = List(4, 8, 12)

There are two changes here:

Happily, both these limitations are slated to go away in a future version. However, right now they are definitely an unnecessary, trivial annoyance when writing Scala.

Extraneous extension methods on Any

Scala adds a bunch of extension methods on every value in your codebase:

@ 1.ensuring(_ > 2)
java.lang.AssertionError: assertion failed
  scala.Predef$.assert(Predef.scala:204)
  scala.Predef$Ensuring$.ensuring$extension2(Predef.scala:316)
  $sess.cmd25$.<init>(cmd25.sc:1)
  $sess.cmd25$.<clinit>(cmd25.sc:-1)

@ 1.formatted("hello %s")
res26: String = "hello 1"

@ 1.synchronized(println("yo"))
yo

It really shouldn't. In my experience, there extension methods are rarely ever used. If someone wants to use them, they can import the functions themselves or write their own extensions. It doesn't make any sense to pollute the method-namespace of every value in existance to add some unused functionality.

Convoluted de-sugaring of for-comprehensions

Scala lets you write for-comprehensions, which are converted into a chain of flatMaps an maps as shown below:

@ val (x, y, z) = (Some(1), Some(2), Some(3))
x: Some[Int] = Some(1)
y: Some[Int] = Some(2)
z: Some[Int] = Some(3)
@ for{
    i <- x
    j <- y
    k <- z
  } yield i + j + k
res40: Option[Int] = Some(6)
@ desugar{
    for{
      i <- x
      j <- y
      k <- z
    } yield i + j + k
  }
res41: Desugared = x.flatMap{ i => 
  y.flatMap{ j => 
    z.map{ k => 
      i + j + k
    }
  }
}

I have nicely formatted the desugared code for you, but you can try this yourself in the Ammonite Scala REPL to verify that this is what the for-comprehension gets transformed into.

This is a convenient way of implementing nested loops over lists, and happily works with things that aren't lists: Options (as shown above), Futures, and many other things.

You can also assign local values within the for-comprehension, e.g.

@ for{
    i <- x
    j <- y
    foo = 5
    k <- z
  } yield i + j + k + foo
res42: Option[Int] = Some(11)

The syntax is a bit wonky (you don't need a val, you can't define defs or classes or run imperative commands without _ = println("debug")) but for simple local assignments it works. You may expect the above code to be transformed into something like this

res43: Desugared = x.flatMap{ i => 
  y.flatMap{ j =>
    val foo = 5
    z.map{ k => 
      i + j + k
    }
  }
}

But it turns out it instead gets converted into something like this:

@ desugar{
    for{
      i <- x
      j <- y
      foo = 5
      k <- z
    } yield i + j + k + foo
  }
res43: Desugared = x.flatMap(i => 
  y.map{ j =>
    val foo = 5
    scala.Tuple2(j, foo)
  }.flatMap((x$1: (Int, Int)) => 
    (x$1: @scala.unchecked) match {
    case Tuple2(j, foo) => z.map(k => i + j + k + foo)
    }
  )
)

Although it is roughly equivalent, and ends up with the same result in most cases, this output format is tremendously convoluted and wastefully inefficient (e.g. creating and taking-apart unnecessary Tuple2s). As far as I can tell, there is no reason at all not to generated the simpler version of the code shown above.

For-comprehensions syntax restrictions

As mentioned above, you cannot have defs, classes, or imperative statements in the generators of a for-comprehension:

scala> for{
     |   i <- Seq(1)
     |   println(i)
     |   j <- Seq(2)
<console>:4: error: '<-' expected but ';' found.
  j <- Seq(2)
^

This is a rather arbitrary restriction, and as far as I can tell doesn't serve any purpose, and forces you to put random _ = prefixes on your statements to make things compile:

scala> for{
     |   i <- Seq(1)
     |   _ = println(i)
     |   j <- Seq(2)
     | } yield j
1
res0: Seq[Int] = List(2)

There really isn't any reason that this shouldn't work out-of-the-box, and convert say:

for{
  i <- Seq(1)
  def debug(s: Any) = println("Debug " + s)
  debug(i)
  j <- Seq(2)
  debug(j)
  k <- Seq(3)
} yield i + j + k

Into

Seq(1).flatMap{ i => 
  def debug(s: Any) = println("Debug " + s)
  debug(i)
  Seq(2).flatMap{ j =>
    debug(j)
    Seq(3).map{ k => 
      i + j + k
    }
  }
}

Abstract/non-final case classes

You can inherit from case classes and extend them with new functionality:

@ case class Foo(i: Int)
defined class Foo

@ Foo(1)
res18: Foo = Foo(1)

@ Foo(1).i
res19: Int = 1

@ class Bar extends Foo(1)
defined class Bar

@ (new Bar).i
res21: Int = 1

You can even declare it an abstract case class to force someone to inherit from it rather than instantiating it directly. If you want your case class to not allow inheritance you should label it final

As far as I can tell, "nobody" does any of this: people don't inherit from case classes, declare their case classes abstract, or rememebr to mark them final. Literally the only programmer I've ever seen making good use of abstract case classes and inheritance is probably Martin Odersky himself. I think we should disallow it, and just force people to use normal classes if they want to inherit from them.

Classes cannot have only implicit parameter lists

This doesn't work:

@ class Foo(i: Int)
defined class Foo

@ new Foo(1)
res50: Foo = $sess.cmd49$Foo@7230510

@ class Bar(implicit i: Int)
defined class Bar

@ new Bar(1)
cmd52.sc:1: no arguments allowed for nullary constructor Bar: ()(implicit i: Int)$sess.cmd51.Bar
val res52 = new Bar(1)
                    ^
Compilation Failed

But this does:

@ new Bar()(1)
res52: Bar = $sess.cmd51$Bar@467de021

This one straddles the line between "Wart" and "Bug", but definitely should be fixed so that a class defined with one argument list doesn't magically sprout two.

Presence of comments affects program logic

Did you know the presence or absence of comments can affect the logic of your program?

@ object Foo{
    def bar(x: Any) = println("Foo#bar(x) " + x)
    def bar = println("Foo#bar")
  }
defined class Foo

@ val baz = 1
baz: Int = 1

@ {
  Foo bar
  baz
  }
Foo#bar(x) 1

@ {
  Foo bar

  baz
  }
Foo#bar

@ {
  Foo bar
  // Wooo!
  baz
  }
Foo#bar(x) 1

@ {
  Foo bar
  // Wooo!

  baz
  }
Foo#bar


@ {
  Foo bar
  // Wooo!
  // Hoo!
  baz
  }
Foo#bar(x) 1

As you can see, this code behaves differently if we have a line between the Foo bar and the baz, unless that line has a line comment on it! When there's no newlines or the newline is filled by a comment, Foo.bar(x: Any) gets called, and when there's a newline not filled by a comment then the other overload Foo.bar gets called.

There are other places in the language syntax where this is the case:

@ {
  class X(x: Int)(y: Int)

  new X(1)(2)

  class Y(x: Int)
         (y: Int)

  new Y(1)(2)
  }
defined class X
res103_1: X = $sess.cmd103$X@6a58eda9
defined class Y
res103_3: Y = $sess.cmd103$Y@136ca935

@ {
  class Z(x: Int)

         (y: Int)
  }
cmd105.sc:3: not found: value y
       val res105_1 = (y: Int)
                       ^
Compilation Failed

@ {
  class W(x: Int)
  // Woohoo
         (y: Int)
  }
defined class W

A full listing of the places where this "comment can change behavior of newlines" can be found in the OneNLMax rule of the ScalaParse grammar. I don't have an immediate answer for what the correct solution is here, but I'm 99.9% sure we should make it so comments like this don't affect the semantics of a Scala program!

Conflating total destructuring with partial pattern-matching

The following example, also from Lincon Atkinson's blog, compiles without warning and fails with an exception at runtime:

@ val (a, b, c) = if (true) "bar" else Some(10)
scala.MatchError: bar (of class java.lang.String)
  $sess.cmd105$.<init>(cmd105.sc:1)
  $sess.cmd105$.<clinit>(cmd105.sc:-1)

The basic problem here is that when Scala sees val (a, b, c) = ..., it doesn't mean which of two things you mean:

  1. Help me extract the values from ..., and help me check that it's a tuple
  2. Help me extract the values from ..., and fail at runtime if it is not a tuple

Currently, it assumes the latter, in all cases.

That makes any sort of "destructuring assignment" unchecked, and thus extremely unsafe.

The above example at least happily fails with an exception, but the following exhibits the same problem, but instead truncates your data silently, losing the 5:

@ for((a, b) <- Seq(1 -> 2, 3 -> 4, 5)) yield a + " " +  b
res107: Seq[String] = List("1 2", "3 4")

Though the following also fails with an exception:

@ Seq(1 -> 2, 3 -> 4, 5).map{case (a, b) => a + " " + b}
scala.MatchError: 5 (of class java.lang.Integer)
  $sess.cmd108$.$anonfun$res108$1(cmd108.sc:1)
  scala.collection.TraversableLike.$anonfun$map$1(TraversableLike.scala:234)
  scala.collection.immutable.List.foreach(List.scala:389)
  scala.collection.TraversableLike.map(TraversableLike.scala:234)
  scala.collection.TraversableLike.map$(TraversableLike.scala:227)
  scala.collection.immutable.List.map(List.scala:295)
  $sess.cmd108$.<init>(cmd108.sc:1)
  $sess.cmd108$.<clinit>(cmd108.sc:-1)

While interpretation #2 makes sense in match blocks and partial-functions, where you expect to "fall through" to the next handler if it doesn't match, it doesn't make much sense in cases like this where there is nowhere to fall through to.

The correct solution would look something like this:

A possible syntax might be using case, which Scala developers already associate with partial functions and pattern matches:

for(case (a, b) <- Seq(1 -> 2, 3 -> 4, 5)) yield a + " " +  b

case val (a, b, c) = if (true) "bar" else Some(10)

This would indicate that you want to perform an "partial fail at runtime" match, and the earlier non-case examples:

for((a, b) <- Seq(1 -> 2, 3 -> 4, 5)) yield a + " " +  b

val (a, b, c) = if (true) "bar" else Some(10)

Could then verify that the pattern match is complete, otherwise fail at compile time.

Conclusion

Most of the warts listed here are not inherent to the "core" of the Scala language: types, values, classes, traits, functions, and implicits. None of them are particularly deep, nor should they be very controversial. This list is obviously neither objective nor comprehensive.

Nevertheless, these warts are annoying far beyond their level of sophistication, and especially pose a barrier to newbies (such as myself, six years ago) who haven't learned to "tune out the noise" and "jump through the hoops" to be able to work with Scala's elegant core.

I don't have the capability to contribute fixes to all of these myself, but hopefully by publishing this I'll be able to raise awareness in the community about such problems, and add pressure so that some day all these sharp corners can be sanded down to reveal Scala's true elegance.

If you have your own favorite warts in the Scala language, let us know in the comments below!


Updated 2017-05-28 2017-05-28 2017-05-27 2017-05-27 2017-05-27 2017-05-27 2017-05-27