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How do sum types make error handling safer (without also adding exhaustive pattern matching) or easier (without adding do notation or macros)?

The point I am trying to make is that adding some features from other languages doesn't provide sufficient value without also adding other features. Adding all of them can make the language significantly more complex that you wonder whether the benefits gained are worth it.

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The problem with omitting language features like this is that the programmer is then forced to implement them by hand in an error-prone way. It's the same issue with Python and tail call optimization. Guido refuses to implement tail call optimization, so the programmer has to implement it herself in terms of a loop. Go refuses to implement sum types, so programmers code them up by hand. In neither case is the code actually clearer.

Since proper error-checking in Go actually requires doing by hand what a pattern match would have done anyway you're not simplifying anything for anyone by omitting them; you're just making it harder for the compiler to provide compile-time checks.

A pattern match is basically a switch statement. Go already provides the latter so the language would not need to become more complicated.

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