Hello, what is the right way to forward varargs?
proc echoWrapper(args: varargs[string, `$`]) =
echo args
echoWrapper "hi", " bye"Ok, how about a bit more useful case, that is forwarding varargs[expr]?
template forwarder(args: varargs[expr]) =
echo argsI have found a method which should work in most of the cases.
macro appendVarargToCall(c: expr, e: expr): expr =
result = c
for a in e.children:
result.add(a)
template echoWrapper(args: varargs[expr]) =
appendVarargToCall(echo(), args)Just another witness of Nim's power!
proc p*(args: varargs[typed, `$`]) =
echo args
proc p*(args: varargs[typed, `$`]) =
echo argsI have never been able to make varargs[typed, `$`] work. I use string instead of typed:
proc p(args: varargs[string, `$`]) =
echo argsUnfortunately, args is displayed as an array and you have to loop on the elements and use stdout.write to get what is expected.
To forward a varargs, this works as the compiler transforms the sequence as a list of parameters:
proc p1(args: varargs[string]) =
echo args
proc p2(args: varargs[string, `$`]) =
p1(args)
p2(1, 2, 3)Thus, adding an element can be done this way:
proc p1(args: varargs[string]) =
echo args
proc p2(args: varargs[string, `$`]) =
p1(@args & "4")
p2(1, 2, 3)There is also a macro unpackVarargs but it is not documented.
Seems like unpack_varargs behaves not exactly as calling directly
import macros
proc p(args: varargs[string, `$`]) = unpack_varargs(echo, args)
p 1, 2, 3
echo 1, 2, 3Produces
["1", "2", "3"]
123Yeah that's what @lscrd meant about having to loop through with stdout.write, if you want to duplicate echo's behaviour
proc p1(args: varargs[string, `$`]) =
for a in args:
stdout.write(a)
stdout.write('\n')
proc p2(args: varargs[string, `$`]) =
p1(@args & "4")
p2(1, 2, 3) # 1234
Hey! Just to check, is there a good way unpack/forward varargs in a recent Nim version, like 2.2.0? (I just noticed that I am actually testing it on 2.0.8 on a Mac.)
For example, I would like to make some bit masks at compile time, which can be used in the rest of the code. Like this:
proc bitMask(bits: varargs[int]): int =
result = 0
...
const
init_portB = bitMask(1,2,5)There is already setBits macro in bitops:
macro setBits(v: typed; bits: varargs[typed]): untyped
var v = 0b0000_0011'u8
v.setBits(3, 5, 7)
doAssert v == 0b1010_1011'u8I would like to reuse this macro. I.e. hide the temporary variable var v in a proc or another macro, which forwards the varargs to setBits:
import bitops
proc bitMask(bits: varargs[int]): int =
result = 0
setBits(result, *varargs)Is it possible to do?
Just to note, of course, this bitMask could be implemented with bitwise operators like this:
proc bitMask(bits: varargs[int]): int =
result = 0
for bit in bits:
result = result or (1.shl bit)I don't really care about this specific case. The important point is whether varargs work in general. Also, in case of setBits, it is a macro that returns untyped. I.e. it works on Nim nodes. So, I wonder if something breaks when you pass typed varargs[int] up the compilation chain to untyped nodes. I guess it should work fine here, because the argument is typed varargs[typed].