How to convert List to LazyList?

Question

How to make a list lazy, i.e. create a LazyList given a usual one? I tried to find suitable method in Scala documentation, but there is no such function.

Jipijapa

list.to(LazyList) - in any case, what is the point? it is already computed. – Scarify

myList.view will create a lazy SeqView so that future traversals will be lazy. – Almsgiver

This looks like an XY problem. Why do you want to convert a List into a LazyList? There's no use of that laziness now – Macedonian

@LuisMiguelMejíaSuárez Lazy computations using the list as input? List(1,2,3,...,100).to(LazyList).map(veryExpensiveComputation).foldLeft(...).takeUntil... – Puffin

@JamesMoore sure, although using view or iterator would have been better in this case. That is why I asked OP what was the end goal. – Scarify

You want to convert a List into a LazyList so that transformations done afterwards have the LazyList semantics (memoized and lazy) – Fermata

Use case: I have a List[java.io.Path] which you want to turn into a LazyList[Json] by reading the files to subsequently do various transformation on the Json that does not depend on the other input files/paths. – Bismarck

Another non XY problem use case: You need to use a third party library that expects only a LazyList – Only

Answer 1

One way would be to unpack the list into a LazyList. For example:

LazyList(List(1,2,3) :_*)

Bismarck

Are you sure this works like you think for very large lists? IOW, how is it different, more preferable, or less desirable than using the .to(LazyList) approach in my answer: https://mcmap.net/q/1564229/-how-to-convert-list-to-lazylist – Misdirect

From your own example, substituting val validChars = LazyList(('A' to 'Z') :_*) works fine. Perhaps you can state what specifically you think is wrong with it? As far as style, it's just different to yours. – Bismarck

Answer 2

To those Asking, "What's the point of wrapping an already computed Seq or List with a LazyList?"

Answer: Inferred type propagation

Example without LazyList which causes an OOME (OutOfMemoryException):

val validChars = //List[Char]
  ('A' to 'Z')
    .toList

val allCodes =
    for {
      a <- validChars
      b <- validChars
      c <- validChars
      d <- validChars
      e <- validChars
      f <- validChars
      g <- validChars
      h <- validChars
      i <- validChars
      j <- validChars
    } yield s"$a$b$c$d$e$f$g$h$i"

Example with LazyList enabling all sorts of stream-like approaches without generating an OOME:

val validChars = //LazyList[Char]
  ('A' to 'Z')
    .to(LazyList)

val allCodes =
    for {
      a <- validChars
      b <- validChars
      c <- validChars
      d <- validChars
      e <- validChars
      f <- validChars
      g <- validChars
      h <- validChars
      i <- validChars
      j <- validChars
    } yield s"$a$b$c$d$e$f$g$h$i"

Another option is to change the first line of the for comprehension to:

a <- validChars.to(LazyList)

Misdirect