nimforum mirror - Hidden generics

mora (orginal) [2015-07-04T15:07:58+02:00] view original

Hi,

Recently I was surprised that a function can take tuple as a parameter, so I started to play with it. It might generate several functions in C like when I use generics. Is there any other hidden logic to use generics?

More details:

If I create a test function with generics and call it with two different type of parameters:

proc test[T](x: T): T =
  result = 10 + x

echo test(1)   # prints 11
echo test(2)   # prints 12
echo test(3.0) # prints 13.0

then the C code will contain two instances: one for int and one for float. Everything is clear.

Then I know about templates and macros. The previous example with template:

template testTemplate(x: expr): expr =
  10 + x

echo testTemplate(1)   # prints 11
echo testTemplate(2)   # prints 12
echo testTemplate(3.0) # prints 13.0

All the 3 cases the code is replaced by the template (the compiler is smart enough to calculate the result, no addition at runtime). I understand this.

So I was surprised to see this (thanks again def, you showed this to me):

proc myPrint(xs: tuple): string =
  result = "("
  for x in xs.fields:
    if result.len > 1:
      result.add(", ")
    result.add($x)
  result.add(")")

echo myPrint((1,2))         # prints (1, 2)
echo myPrint((3,4))         # prints (3, 4)
echo myPrint(("A","B","C")) # prints (A, B, C)

This function takes a tuple as parameter, and in this case generates a function which takes (int, int), and a function which takes (string, string, string). Moreover, the for cycle is unwrapped by the compiler. I like this, but this feels like black magic. Is there somewhere more info about this? What else can I do like this (e.g. proc, seq, array)?

Thank you, Peter

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