Using local dependencies with Go modules

Go 1.16, released this week, deprecated GOPATH. I used it extensively to develop my libs before publishing them, but now I’m forced to convert them to modules, which work with remote repos by default. However, there’s a way to work with local dependencies.

As I found here, we can instruct Go toolchain to search for a local repo, instead of a remote one by using this command:

go mod edit -replace github.com/username/repo=../repo

This changes the go.mod file. Now, to clean up the go.sum file, run:

go mod tidy

After that, you should be able to use a local dependency just like the old GOPATH days.

The downloaded files are still cached, though. To finally clean the entire cache, run:

go clean -cache -modcache

Modules eventually needed by other applications will be downloaded again when due.

Downcasting boxed errors in Rust

Today I was trying to write polymorphic error handling in Rust, something achievable with dynamic_cast in C++, and incredibly easy in Go. In Rust, of course, it’s overcomplicated.

After a couple frustrating hours of searching with no success, I stumbled upon the downcast_ref method of the Error trait, which finally allowed me to write what I wanted. Oddly enough, I couldn’t find this solution anywhere.

Note that the String concrete type cannot be retrieved.

Playground link here.

Cleaning up a Git repository

Sometimes, after you mess up too much with your Git repository, it ends up with a couple useless objects, which are no longer referenced anywhere.

I created this alias a while ago, collected from sources I don’t remember anymore, to perform a cleanup in the repository. It’s particularly useful right after a git fetch:

alias gitcleanrepo='git fetch origin --prune && git -c gc.reflogExpire=0 -c gc.reflogExpireUnreachable=0 -c gc.rerereresolved=0 -c gc.rerereunresolved=0 -c gc.pruneExpire=now gc "$@"'

Inserting auto IDs in Oracle

When dealing with tables with an auto-incremented ID field, Oracle requires an special syntax if you’re about to insert data manually. You must directly call the name of the sequence of that table, followed by a special field:

INSERT INTO person (id_person, name)
VALUES (sq_id_person.NEXTVAL, 'John')

Note that the name of the sequence is actually arbitrary.

Validating lambda signatures in C++17

WinLamb, as the name itself implies, is heavily based on lambda functions. While searching a way to enforce the lambda signatures in the upcoming C++17 release, I received an incredible answer on StackOverflow:

#include <functional>
#include <type_traits>
	
template<typename F>
auto foo(F&& func) noexcept ->
	std::enable_if_t<
		std::is_same_v<
			decltype(std::function{std::forward<F>(func)}),
			std::function<void(int)>
		>, void
	>
{
	func(33);
}

Up to this point, I was completely unaware of the existence of trailing return types in function declarations.

The validation is extremely restrict, serving my purposes perfectly. And it seems that the upcoming C++20 concepts feature can simplify it quite a lot.

SFINAE and enable_if

Today, while researching for the upcoming C++17 version of WinLamb, I was rethinking the idea of restricting the allowed types on a template argument. That’s an SFINAE thing, scary to many people – including me sometimes, in my love-hate relationship with advanced C++ stuff.

The thing is that I ended up writing a working piece of code using std::enable_if to restrict a template argument, and I’m quite proud of it.

#include <type_traits>

struct One { int num; };
struct Two { int num; };
struct Tre { int num; };

template<
  typename T,
  typename = std::enable_if_t<
    std::is_same_v<T, One> ||
    std::is_same_v<T, Two>
  >
>
void foo(T& o) { ++o.num; }

int main() {
  One o;
  foo(o);
  return 0;
}

In the code above, One and Two classes will work with foo(), whereas Tre will not compile, despite being exactly like the others.

CSS folded poster effect

Yesterday I just stumbled across this incredible CSS work, which makes a picture look like a folder poster on Reddit. The CSS was somewhat overengineered, so I removed a bunch of stuff and fixed some parts, and the result is now pretty usable.

The border is set by the padding attribute, and may need to be adjusted according to the image size. The image size itself cannot be properly set, or the effect goes nuts – if you want to resize the image, resize the image file itself.

I hope to use it somewhere, because it’s really cool. Props to the dude who did it.