I'm trying to perform the equivalent of the eval method from Python in nim.
I was under the impression that parseStmt from the macros package should help me with this, but I'm facing a compilation issue that I don't understand.
import macros
echo parseStmt("1 + 2")
I would have expected this to print 3 when executed, but instead the compilation complains that
Error: request to generate code for .compileTime proc: $
I found this thread, and the examples there work, and following this, I was able to make the following program that works as I would expect:
import macros
import strformat
macro eval(value: string): untyped =
result = parseStmt fmt"{value}"
echo eval("1+2")
But I don't undertand why it needs to be written in this way at all. If I inline the statement, let value = "1 + 2"; echo parseStmt fmt"{value}", I get the same compile error as above.
Also, why is parseStmt value different from parseStmt fmt"{value}", in the context of the eval macro above?
What am I missing here?
Thank you in advance for any clarifications!
Unlike Python which is an interpreted language, Nim is compiled. This means that all code is parsed and turned into machine code on compile-time and the program that you end up with doesn't really know anything about Nim at all (at least as long as you don't import the Nim compiler as a module, which is possible). So parseStmt and all macro/template expansion stuff in Nim is done completely during compilation. The error, although maybe a bit hard to read, is trying to tell you that what was passed to $ (which is the convert-to-string operator in Nim, and called by echo on all its arguments) is a compile-time thing that can't be used on runtime. In this case it's because parseStmt doesn't return "3", it returns something like NimNode(kind: nnkIntLit, intVal: 3), and the NimNode type is only available during compile-time. Nim however allows you to run code on compile-time to return other code, this is what a macro does. The eval macro you wrote there takes value which is any statement that resolves to a string during runtime, passed as a NimNode. This is also why result = parseStmt value won't work in your case, because value is not yet a string, but could be something like reading a string from standard input which would result in a string during runtime. However the use of strformat here is a bit confusing and overkill. If you change your code to:
import macros
macro eval(value: static[string]): untyped =
result = parseStmt value
echo eval("1+2")
It will work just fine. This is because we have now told Nim that value needs to be a static i.e. known during compile-time. In this case the string literal "1+2" is obviously known at compile-time, but this could also be a call to a compile-time procedure, or even the output of staticRead which reads a file during compilation.
As you can see Nim is very powerful, but the barrier between compile-time and run-time can sometimes be a bit confusing. Note also that your eval procedure doesn't actually evaluate anything at compile-time, it simply returns the Nim code 1 + 2 so your code ends up being echo 1 + 2. If you want to actually run the code at compile-time you might want to look into compile-time procedures.
Hope this helps shed some light on your issue.