Hello,
First of all, thanks a lot for Nim ! That's the best python-ish compiled language I found (and trust me, I searched !)
I'm coming from scripted languages, and I'm new to the macros world. But I'm used to metaprogramming thanks to python. From my basic understanding, macros modify the AST.
I found this interesting blog post explaining metaprogramming in nim (https://dlesnoff.github.io/nimProgramming-blog/blogPosts/macroTutorial.html) and wanted to try it on nim 2.2.6
At the beginning, every template example works, but I get no result from every macro example.
For example this one :
macro timesTwo(statements: untyped): untyped =
for s in result:
for node in s:
if node.kind == nnkIntLit:
node.intVal = node.intVal*2
timesTwo:
echo 1 # 2
echo 2 # 4
echo 3 # 6
dumpTree:
echo 32
# Outputs this :
StmtList
Command
Ident "echo"
IntLit 32But nothing prints on my screen, I tried the playground with different versions of nim such as 2.2.0 or 1.6.8 but nothing happens.
It seems I'm missing something here, can somebody please help me ?
macro timesTwo(statements: untyped): untyped =
result = statements # <- result is empty
for s in result:
for node in s:
if node.kind == nnkIntLit:
node.intVal = node.intVal*2Interesting, I did not think someone would read my blog post.
Thanks, I'll fix the example. Sadly, my blog post doesn't cover much and has not been updated for a while.
It misses hygienic variables, bindSym, … Among free teaching materials I would recommend Kiloneie's Youtube channel.
I highly recommend reading Araq's book for examples with idiomatic code.
At the beginning, every template example works, but I get no result from every macro example.
I fixed the timesTwo examples both in the english and french versions. Please tell me if you notice something else wrong. I will surely do a complete pass to fix English writing and add further informations.
Thanks for your quick reply to both of you !
That works for me and I get results @tommo, thanks
Et merci beaucoup @dlesnoff pour la mise à jour du post, je l'ai découvert indirectement grâce à HN. Je vais regarder la suite des exemples sur les macros et te faire un retour au besoin. Ensuite la chaine de Kiloneie dans un premier temps et enfin le livre d'Araq ;)
That's a warm welcome in the nim community :)
There are only 2 little things to correct, everything else is working.
1°) This template produces only 1 "Hello World" and honestly I don't know why :
template doWhile(conditional, loop: untyped) =
loop
while conditional:
loop
var i = 10
doWhile i < 10:
echo "Hello World"
i.inc2°) The definition of the macro typeMemoryRepr misses an import
import std/strutils # <-- because of capitalizeAscii
macro typeMemoryRepr(typedef: untyped): untyped =Thanks again, this discussion helped me grasp nim in a better way.
And I'm sure your blog post will help other people too
For number 1 I think you might've meant to do var i = 0, because the expected outcome of a do..while loop is indeed to only run once in the case where i starts out higher than the conditional. That being said there is a little trick that makes the traditional doWhile template better. It's important to remember that templates essentially just copy-pastes code. This means that they can quickly make your code size quite large. Imagine your loop body contained a lot of code, with your doWhile you make two copies of all of it! Furthermore you probably expect things like break or continue to work in a loop. But since the first loop statement is outside the while block they will cause a compilation-error, or even worse break out of an outer loop on accident! The solution is fortunately simple:
template doWhile(conditional, loop: untyped) =
var c = true
while c:
defer: c = conditional
loopThe loop code is now only pasted once, and with the defer statement we are sure the conditional is re-evaluated even in the case you call continue from within your loop.
Meta-programming can be a bit tricky to get right, there are a couple of gotchas around. But when you get it right it's incredibly powerful!
Wrote up a small example showing the difference between the two procs.
Imagine we wanted to only run our code when i was even, and we came up with the (slightly strange) code:
for msg in ["Hello even world", "Goodbye even world"]:
var i = 1
doWhile i < 10:
if i mod 2 != 0:
inc i
continue
echo i, ": ", msg
i += 2Now, there are better ways of doing this, but if we imagine that our "work" could change i this makes sure we only even run our body when i is even, and skips to the next even number.
With the original template this code throws the error:
Error: invalid control flow: continue
for msg in ["Hello even world", "Goodbye even world"]:
var i = 1
doWhile i < 10:
if i mod 2 != 0:
inc i
continue
echo i, ": ", msg
i += 2With the upgraded doWhile template both cases run fine and produces the expected result.
Thanks!
1) If that was a while procedure, we would not enter the loop as i=10 and we check whether i < 10. Now, with a doWhile, we execute the code once, print, and increase the variable i. Checking the conditional, we have 11 not strictly lower than 10, so we do not execute the code again.