mattaylor / elvis Goto Github PK

We only need the last line with the typeclass. It's equivalent to the top three. https://nim-lang.github.io/Nim/manual.html#generics-type-classes

I've been working with a C library, and the try/catch mechanism doesn't work well for digging into structs.

For example, I call function that returns a ptr.

  let draw: ptr tm_creation_graph_draw_call_data_t = render_component_api.draw_call(...)

  if ?draw.shader: 
    # do stuff

If draw is nil then I'll get a crash because the access violation is coming from C and not from Nim. My work around is to use my conditional access macro that does checks at each step.

  if ?draw.?shader:
    # do stuff

Thanks for the changes! I was going to push a PR, but you saved me the trouble. :)

I'm currently, trying to figure out how to chain .? so we can do:

var val = obj.?data.?val

I'm looking at https://nim-lang.github.io/Nim/manual_experimental.html#special-operators-dot-operators or https://nim-lang.github.io/Nim/manual_experimental.html#pattern-operators-the-nimstar-operator but I haven't figured it out yet.

Current conditional access templates fail to compile when matching function chains with extra args
The following fail to compile...

let tab = { "one": "uno" }.toTable
check(tab.?getOrDefault("one") == "uno") 
check(tab.?getOrDefault("two") == "")
check(tab.?getOrDefault("two", "unknown") == "unknown")

I'm doing interop with C, and I have lots of structures that have pointer values. I want to use a conditional access operator in combination with a truthy operator like this:

  if ?data?.shader:
    # use the data.shader

But the current implementation expects shader to be a proc. Should I create a new operator for my use case or do you think ?. could be modified to handle this?

Not quite sure what is going on, but I do have a minimal reproducible example:

import elvis

var a: seq[int] = @[]
echo ?(a[0])

type Story* = object
var stories: seq[Story]
echo ?(stories[0])

This prints

false
true

which is pretty counter intuitive, as I'd expect the second echo to print false as well.

https://play.nim-lang.org/#ix=3MaY

The implementation uses compile: https://nim-lang.github.io/Nim/system.html#compiles%2Cuntyped, which always returns true if val is a nillable type, but we want to actually check not val.isNil at runtime.

This will produce the expected results for nillable types.

# true if not isNil()
template truthy*[T](val: ref T): bool = not val.isNil
template truthy*[T](val: ptr T): bool = not val.isNil
template truthy*(val: pointer): bool = not val.isNil

?. has lower precedence than == https://nim-lang.github.io/Nim/manual.html#syntax-precedence

If we wanted to do something like:

if d?.val == 10:
  echo "foo"

We would need to wrap the expression in parentheses:

if (d?.val) == 10:
  echo "foo"

If we change the operator to .? then this works:

if d.?val == 10:
  echo "foo"