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The short answer, devoid of some interesting nuance, is that C++’s template system is similar to an untyped functional programming language, so the errors you g

by oddity 8y ago

The short answer, devoid of some interesting nuance, is that C++’s template system is similar to an untyped functional programming language, so the errors you get are the compiler’s attempt to make sense of the large tree of nonsense it couldn’t reduce. C++ also suffers from years of backwards-compatibility twister creating abundant cases of ambiguity if you do something wrong. The compiler will do it’s best, but if your code suddenly parses wildly differently because your semicolon is missing, oh well. Rust is far simpler in that regard, so there are fewer instances where your code might accidentally make sense to the compiler if you have an error.

bluejekyll 8y ago
> Rust is far simpler in that regard, so there are fewer instances where your code might accidentally make sense to the compiler if you have an error. I say this from a place of love for Rust, but the first time you run into a massive generic type tree error thats root cause is a type parameter three levels deep not implementing Send (or some other trait) it takes your breath away. There are some great jokes about this in Deisel and Futures for example. It makes Java generics look like a baby just starting to craw. The error messages are good, you just need to go grab a coffee while you read through the book on type theory that’s been dumped to your screen.
- marcosdumay 8y ago
  Similar stuff happens on Haskell too. I guess that since the type system is an unityped logic programming language, it is an even worse place than C++ templates. It is a smaller problem on practice only because the type declarations are much shorter programs than C++ templates.
fmap 8y ago
Let me just stress the first point, because that's exactly where Rust and C++ differ: C++ templates really are untyped. There is no way to check whether a template definition is correct. A C++ compiler has to first expand the templates ("complete monomorphisation") and then perform type checking. Any errors you get will be in terms of the template instantiations, not your original code. Unlike C++, Rust has a type system for the full language. In particular, traits are type checked once and you get errors in terms of the code you wrote yourself. There are other compilers that use complete monomorphisation, such as the MLton compiler for Standard ML, where you don't hear horror stories about terrifying error messages because your code was checked before being specialized. I want to stress that this is a terrible design decision in terms of usability, because it is incredibly attractive from an implementation perspective. Parametric types are a delicate issue in an imperative language and always end up rejecting some perfectly fine programs. Monomorphisation both eliminates this issue and potentially leads to more efficient code, but your type errors will suffer. I could say more about this - there are more trade-offs involved - but actually working with template heave C++ code is a better argument than anything I could say. ...That said, the examples in the blog post actually don't use any complicated machinery, and the quality of the compiler errors just comes down to very good engineering on the part of the Rust development team. :)

by oddity 8y ago

bluejekyll 8y ago

> Rust is far simpler in that regard, so there are fewer instances where your code might accidentally make sense to the compiler if you have an error. I say this from a place of love for Rust, but the first time you run into a massive generic type tree error thats root cause is a type parameter three levels deep not implementing Send (or some other trait) it takes your breath away. There are some great jokes about this in Deisel and Futures for example. It makes Java generics look like a baby just starting to craw. The error messages are good, you just need to go grab a coffee while you read through the book on type theory that’s been dumped to your screen.

marcosdumay 8y ago
Similar stuff happens on Haskell too. I guess that since the type system is an unityped logic programming language, it is an even worse place than C++ templates. It is a smaller problem on practice only because the type declarations are much shorter programs than C++ templates.

fmap 8y ago

Let me just stress the first point, because that's exactly where Rust and C++ differ: C++ templates really are untyped. There is no way to check whether a template definition is correct. A C++ compiler has to first expand the templates ("complete monomorphisation") and then perform type checking. Any errors you get will be in terms of the template instantiations, not your original code. Unlike C++, Rust has a type system for the full language. In particular, traits are type checked once and you get errors in terms of the code you wrote yourself. There are other compilers that use complete monomorphisation, such as the MLton compiler for Standard ML, where you don't hear horror stories about terrifying error messages because your code was checked before being specialized. I want to stress that this is a terrible design decision in terms of usability, because it is incredibly attractive from an implementation perspective. Parametric types are a delicate issue in an imperative language and always end up rejecting some perfectly fine programs. Monomorphisation both eliminates this issue and potentially leads to more efficient code, but your type errors will suffer. I could say more about this - there are more trade-offs involved - but actually working with template heave C++ code is a better argument than anything I could say. ...That said, the examples in the blog post actually don't use any complicated machinery, and the quality of the compiler errors just comes down to very good engineering on the part of the Rust development team. :)