options.flatMap doesn't work with sugar.=>

[GULPF]

Example

import options, sugar

var x = some(1)
echo x.map(proc(y: int): int = y + 1) # ok
echo x.map(y => y + 1) # ok
echo x.flatMap(proc(y: int): Option[int] = some(y + 1)) # ok
echo x.flatMap(y => some(y + 1)) # Error: cannot instantiate: 'B'

Additional Information

$ nim -v
Nim Compiler Version 1.4.0

[momeemt]

Hello.
It seems that the => macro generates a procedure with all argument and return value types set to auto type.
However, I tried and found that the current implementation of the auto type implicitly generates a generic routine (source) that automatically sets the Option[T] type to It was not possible to get the Option[T] type automatically.
The same error seems to occur in another example.
I suspect that it is not due to the implementation of the => macro, but due to the inability to get the auto type.

And I don't think this is a problem that can be solved by implementing the => macro.

[schneiderfelipe]

Hi @GULPF, a possible solution is to use the do notation:

import options

var x = some(1)
echo x.map(proc(y: int): int = y + 1) # ok
echo x.map do (y: auto) -> auto:
  y + 1  # ok
echo x.flatMap(proc(y: int): Option[int] = some(y + 1)) # ok
echo x.flatMap do (y: int) -> auto:
  some(y + 1)  # ok

Unfortunately, you have to specify at least y: int as explained by @momeemt. It would be awesome to have parameter types be inferred from the function, though!

[dom96]

IMO if you're going to use the do notation you might as well just write out the full proc (the do notation should be removed from the language :)). Here is a better workaround:

echo x.flatMap((y: int) -> Option[int] => some(y + 1))

[schneiderfelipe]

IMO if you're going to use the do notation you might as well just write out the full proc (the do notation should be removed from the language :)). Here is a better workaround:

echo x.flatMap((y: int) -> Option[int] => some(y + 1))

I'm embarrassed, I didn't know I could use -> together with =>! 😆