Hello,
I have a question concerning generics / function overloading. Say I have a
(somewhat silly for demonstration purposes) "module A". The main function
`quotedPair` takes an argument and returns a tuple containing the argument
itself as well as some sort of "quoted" form of the argument which is generated
using a `quote` function. I provide an implementation of `quote` for strings
but I want to give users of my module the possibility to extend `quotedPair`,
hence `quotedPair` is a generic function.
# moda.nim
proc quote*(s: string): string =
"`" & s & "`"
proc quotedPair*[T](x: T): (T, string) =
(x, x.quote)
Run
But if I try to use module A and extend it with an implementation for integers,
it does not work:
# modb.nim
import moda
proc quote(s: int): string =
# reusing the quote implementation for strings here
quote($s)
# this works
echo quotedPair("hello")
# this is a compile time error
echo quotedPair(1)
Run
I get a compile time error, although I provide an implementation of `quote` for
integers.
../moda.nim(8, 10) Error: type mismatch: got <int>
but expected one of:
proc quote(s: string): string
first type mismatch at position: 1
required type for s: string
but expression 'x' is of type: int
Run
But what I don't understand: As soon as I add _some_ generic version of `quote`
to module A (that does not even has to match `int`) the code _does_ compile.
Like so:
# moda.nim (modified)
# this doesn't match for `x: int`!
proc quote*[T](x: seq[T]): string = ""
proc quote*(s: string): string =
"`" & s & "`"
proc quotedPair*[T](x: T): (T, string) =
(x, x.quote)
Run
What is happening here and is that expected? Also, could I solve my problem
without the generic (dummy) implementation, maybe somehow 'annotating' the
`quote` function to be extendable?
By the way, I spotted this technique in Nim mustache. There is a `Context` type
to which "values" can be added. If the value is not the correct type `Value`,
it gets cast automatically, see:
<https://github.com/soasme/nim-mustache/blob/master/src/mustachepkg/values.nim#L149>
Nim mustache itself provides some implementations of `castValue`, but they are
talking explicitly about writing your own implementations for custom types.
This only seems to work because there are some generic implementations of
`castValue` that are provided by Nim mustache.
