Nim: Return Table from Procedure - nim-lang

Question

I've been trying to write a simple lexer in Nim and I need to my Procedure to return a Table. I've been having some issues with making this happen, here is my code:
main.nim
# Code
import lexer
echo lexer.lex("hi")
lexer.nim
import tables
proc lex*(code: string): Table =
let variable = to_table({"1": "hi"})
return variable
Every time I try compile the code (nim c -r --outDir:"../bin" "main.nim"), I get the following error:
/path/to/main.nim(5, 15) template/generic instantiation of `lex` from here
/path/to/lexer.nim(3, 26) Error: cannot instantiate 'Table[A, B]' inside of type definition: 'lex'; Maybe generic arguments are missing?
I've probably made some simple error, but I have wasted too much time on this, so any help is appreciated.
Operating System: Solus
Nim Version: 1.2.0

Answer 1

There are several issues with the code but lets start trying to decipher the compiler error: Table[A, B]' inside of type definition: 'lex'; Maybe generic arguments are missing?.
Nim has type inference but lets say it is forward declared and fails at the first moment something is unknown. In this case, the compiler is going through your main.nim file and sees that you want to invoke a proc with the following signature:
proc lex*(code: string): Table
That's all it sees, and since Table is a generic type, it fails instantiation of the return type, because it doesn't know what the generic types A or B should be, based on just the proc signature. That's what the compiler is complaining about. You could argue the compiler could try to look into the proc and figure out the specific type of the generic signature, but this requires more work on part of the compiler and would slow down compiling times, a metric the Nim community cares a lot about.
There are two ways to fix that, the simple one, which is declaring the types in the proc signature, and the difficult one, which requires you to specify the types at the caller. The former is a matter of changing the proc signature to this:
proc lex*(code: string): Table[string, string]
The latter one is to specify the type at the caller, which means changing your main.nim code to this:
echo lexer.lex[Table[string, string]]("hi")
Unfortunately this second solution won't compile either because the main.nim module doesn't now anything about the Table type: Error: undeclared identifier: 'Table'. In order to keep using this solution you would need to either export the Table type from lexer.nim adding an export statement to that module, or importing the tables module in main.nim (and any other future calling modules).

Answer 2

You need to be explicit in the type returned by the proc. As Grzegorz says, changing to:
proc lex*(code: string): Table[string, string] =
is enough. No need to modify anything in the main.nim code.
Two minor things to take into consideration:
Nim procs already init a result variable with the return type, which is implicitly returned, so your proc can be rewritten as:
proc lex*(code: string): Table[string, string] =
result["1"] = "hi"
No need to namespace lex proc in main, unless there are name collisions:
import lexer
lex("hi")