FAQ

This lists some commonly asked questions regarding the content of the book.

Compilation errors when < is used

With the latest versions of Nim (for example Nim 1.2.6) you will see compilation errors when compiling some of the samples in the book.

For example, when compiling the database.nim Chapter 7 file, you will see something like:

Tweeter/src/database.nim(82, 17) Error: type mismatch: got <int>
but expected one of:
proc `<`(x, y: int): bool
  first type mismatch at position: 2
  missing parameter: y
proc `<`(x, y: int64): bool
  first type mismatch at position: 2
  missing parameter: y
22 other mismatching symbols have been suppressed; compile with --showAllMismatches:on to see them

expression: <len(usernames)

This is because starting at Nim 1.2.0, a few procedures that were used in Nim in Action have been removed. In particular the unary < operator. When working through the book I would recommend downgrading to Nim 1.0.6 (or a newer patch if one exists, any 1.0.x should work). The easiest way to do this is using choosenim (if you have it), just run choosenim 1.0.6, you might also be able to use your favourite system package manager to get an older version of Nim, or when all else fails you can download 1.0.6 from https://nim-lang.org/download/nim-1.0.6.tar.xz (Linux/macOS/BSD) and https://nim-lang.org/download/nim-1.0.6_x64.zip (Windows x64).

Confusing use of asyncCheck in Listing 3.26

A user named nsfw_ asked this on IRC: https://irclogs.nim-lang.org/28-08-2017.html#21:40:49.

This refers to the asyncCheck used for the connect call, on line 54:

...
var socket = newAsyncSocket()

asyncCheck connect(socket, serverAddr)

var messageFlowVar = spawn stdin.readLine()
...

This is in fact a slight bug in the implementation. The execution of the code will continue without waiting for the socket to be connected. But data will not be sent to the socket unless a line from stdin is read, so for most practical uses of this application it is not a problem.

The best way to solve this would be to use await, but in order to do that the code needs to be wrapped in an async procedure (as await is not allowed in the global scope). The connect procedure also performs two actions, which would need to be separated. It connects to the server and then checks for messages forever.