Template findFirst fails to compile, proc with same code works fine

[gryphis]

Hi
I wrote a template findFirst() to find the first element in a seq[T] which fulfills a predicate - minimal source attached.
Compilation of template code raises:

$ nim c -r nimbug.nim 
Hint: used config file '.../.choosenim/toolchains/nim-1.6.4/config/nim.cfg' [Conf]
Hint: used config file '.../.choosenim/toolchains/nim-1.6.4/config/config.nims' [Conf]
.............................................................
.../nimbug.nim(31, 21) template/generic instantiation of `findFirst` from here
.../nimbug.nim(16, 18) Error: type mismatch: got <RegistryKey>
but expected one of:
proc none(T: typedesc): Option[T]
  first type mismatch at position: 1
  required type for T: typedesc
  but expression 'T' is of type: RegistryKey
proc none[T](): Option[T]
  first type mismatch at position: 1
  extra argument given

expression: none(T)

If template code is copied to a "proc" and all "T" replaced by "RegistryKey" (the type), it works well. You can simply check that by changing a const "failure" in the source.

I work with nim-1.6.4

Thanks for help!

Source:
nimbug.zip

[beef331]

Well there are two issues here, the generic none(T) and also the using result inside a template, templates do not have result, so the above code is not proper. Slight work around ahead.

import options

type
  RegistryKey = object
    key : string
    val : string
var regKey = @[
  RegistryKey(key:"ica.sflogin.debug", val:"true"),
]
template findFirst[T](s: seq[T], pred: proc(x: T): bool): Option[T] =
  var res = none(typeof T)  # if no items satisfy the predicate
  for x in s:
    if pred(x):
      res = some(x)
      break
  res

proc getval(searchKey: string): Option[string] =
  let found = regKey.findFirst(proc (rk: RegistryKey): bool = rk.key == searchKey)
  if found.isNone: none(string)
  else: some(found.get().val)

when isMainModule:
  assert(getval("strange") == none(string))
  assert(getval("ica.sflogin.debug") == some("true"))

[gryphis]

Great thanks for your quick help!! Andi Kahnt Am 02.03.2022 02:25 schrieb Jason Beetham:

Well there are two issues here, the generic none(T) and also the using result inside a template, templates do not have result, so the above code is not proper. Slight work around ahead. import options type RegistryKey = object key : string val : string var regKey = @[ RegistryKey(key:"ica.sflogin.debug", val:"true"), ] template findFirst[T](s: seq[T], pred: proc(x: T): bool): Option[T] = var res = none(typeof T) # if no items satisfy the predicate for x in s: if pred(x): res = some(x) break res proc getval(searchKey: string): Option[string] = let found = regKey.findFirst(proc (rk: RegistryKey): bool = rk.key == searchKey) if found.isNone: none(string) else: some(found.get().val) when isMainModule: assert(getval("strange") == none(string)) assert(getval("ica.sflogin.debug") == some("true")) -- Reply to this email directly, view it on GitHub [1], or unsubscribe [2]. Triage notifications on the go with GitHub Mobile for iOS [3] or Android [4]. You are receiving this because you authored the thread.Message ID: ***@***.***>

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