Mirror of forum.nim-lang.org

6478 :: Help, I can't return an openarray

• snej (orginal) [2020-06-23T19:41:27+02:00] view original

  I'm implementing a lightweight sequence-like type. I want it to have a toOpenArray proc, like seq and string do, so my type will work with any function that takes an openarray. But the compiler refuses to consider it: as soon as it sees the return type it shuts me down.

  proc toOpenArray(s: MySeq): openarray[byte] =
          ^ Error: invalid type: 'openArray[byte]' in this context: 'proc (): openArray[byte]' for proc
  Run

  I s'pose this is part of the "openarray only works as a parameter type" limitation. Is there any workaround; a pragma or something? Or are only "magic" procs allowed to do this?

---

• juancarlospaco (orginal) [2020-06-23T20:09:32+02:00] view original

  Return an array?.

---

• snej (orginal) [2020-06-23T22:26:10+02:00] view original

  An array type has a fixed size. My type is variable-sized. (That's the whole reason openarray exists, right?)

---

• shashlick (orginal) [2020-06-23T22:40:44+02:00] view original

  openarray exists to create procs that accept both fixed sized array as well as variable sized seq. toOpenArray is compiler magic. I don't think you can do this with a custom type.

---

• mratsim (orginal) [2020-06-24T00:03:25+02:00] view original

  To return openarray you need to use a template. Openarrays cannot exist apart as parameter but as templates are plain code substitution they allow you to workaround this rule.

---

• snej (orginal) [2020-06-24T01:34:42+02:00] view original

  Thank you! That did the trick. 🥳

---