Yeah, it's easy: once you know a dozen languages or so the quick reference like this one is all you really need to learn most languages, at least to the point of comfortably writing everyday code. I say "most" languages, because this kind of reference doesn't work for languages based on completely different semantics than the languages you know: APL and J are the primary examples for me (EDIT: also, Forth. But not Factor or Cat or Joy... dunno why exactly).

Anyway, this is great for polyglot programmers, who would be bored to tears by "beginners guide" to just about any language and who know enough about how programming languages work. It's probably of very little help to programmers who know only a very few languages and/or for those who didn't study programming language design, though.

In my case I successfully used this site (and some other resources, as well) to get up to speed with Haxe, Nim, LiveScript and Lua, but then again, learning new languages is something I do all the time (https://klibert.pl/articles/programming_langs.html) for years now, so I may not be the best example :)

To pick up the basics of a language similar to one you already know? Sure. I used this site to write something in Nim just a few days ago.

Do I know the conventions or idioms? No. Do I know the advanced features of Nim? I sincerely doubt it. But I know Python, and seeing the syntax written out is enough for me to correct my informed guesses in how to write it.

I doubt I could use it to pick up Haskell or a language whose paradigm I was unfamiliar with though.

I knew ruby, java, and javascript

I wanted to contribute to a golang project

I spent an hour going through the whole page on Go and then used it as a reference during my actual code writing.

I also use that site as an intro to new languages because of a consistent style of content.

I use it as a quick syntax reminder when I'm dipping into a language I haven't used for a while.

I use this + Rosetta code to get a grasp of a language. I would be able to write something big in the language, but learnxinyminutes.com and Rosetta code allow me to translate bits of code or understand a language I otherwise wouldn't know.

As a reference, it seems quite useful. Not sure how well this would be suited to learning a new language though. To really learn programming one has to invest several years, of course [0].

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[0]: http://norvig.com/21-days.html