Modules

Nim supports splitting a program into pieces by a module concept. Each module needs to be in its own file and has its own namespace. Modules enable information hiding and separate compilation. A module may gain access to the symbols of another module by the import statement. Recursive module dependencies are allowed, but are slightly subtle. Only top-level symbols that are marked with an asterisk (*) are exported. A valid module name can only be a valid Nim identifier (and thus its filename is identifier.nim).

The algorithm for compiling modules is:

• Compile the whole module as usual, following import statements recursively.
• If there is a cycle, only import the already parsed symbols (that are exported); if an unknown identifier occurs then abort.

This is best illustrated by an example:

# Module A
type
  T1* = int  # Module A exports the type `T1`
import B     # the compiler starts parsing B
proc main() =
  var i = p(3) # works because B has been parsed completely here
main()

# Module B
import A  # A is not parsed here! Only the already known symbols
          # of A are imported.
proc p*(x: A.T1): A.T1 =
  # this works because the compiler has already
  # added T1 to A's interface symbol table
  result = x + 1