I keep hearing good things about it. I might try some next year, mainly out of interest and to see how proper oop works.
With just a bit of code you could do the same with Closure.
Try Squeak :-)
From just a bit googling before pharo.org seemed also decent? To much other stuff this moment anyway.
I don't know Pharo, I'll check it out. I suggested Squeak because it's the only Smalltalk version I've played with
Pharo is very mature and it's community is quite active, you can even do some web development with it :D
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