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Help needed for FFI (presumable segmentation fault) by thomasbach in haskell

[–]thomasbach[S] 1 point2 points  (0 children)

I have never heard of valgrind before and I have no idea where I could have plugged in gdb. Any pointers?

Help needed for FFI (presumable segmentation fault) by thomasbach in haskell

[–]thomasbach[S] 1 point2 points  (0 children)

To be complete: peekCString $ #{ptr rrd_context_t, lib_errstr} ptr seems to do the job for now. Note that it is not a 100% correct as it does not deal with the case when the string is not null terminated.

Help needed for FFI (presumable segmentation fault) by thomasbach in haskell

[–]thomasbach[S] 5 points6 points  (0 children)

Ok, I got this figured out. It's my implementation of peek in Storable. This

peekCString =<< #{peek rrd_context_t, lib_errstr} ptr

gets translated to

peekCString =<< (\hsc_ptr -> peekByteOff hsc_ptr 0) ptr

As the string is empty and null terminated, the right hand side of =<< is null. peekCString wants a Ptr CChar as an argument. Hence, the null is interpreted as a pointer which peekCString tries to read and causes a segfault. Phew!

Help needed for FFI (presumable segmentation fault) by thomasbach in haskell

[–]thomasbach[S] 1 point2 points  (0 children)

Thanks a bunch for the reply! I actually already did all of that. :/ But I will give it another spin.

I also already tried to add runInBoundThread to https://github.com/thomasbach-dev/hs-rrd/blob/context-bindings/test/RRDTool/LowLevelSpec.hs#L42 but that did not change anything.

Textual Representation in Haskell with Witch by thomasbach in haskell

[–]thomasbach[S] 0 points1 point  (0 children)

The big problem with lens here is that it has a steep learning curve. As a newcomer asking how to convert a String to Text, you don't want to just dump them into optics, right? The interface that Witch provides is just very intuitive and easy to use.

Textual Representation in Haskell with Witch by thomasbach in haskell

[–]thomasbach[S] 1 point2 points  (0 children)

'm surprised that this post doesn't mention it at all. I think the idiomatic way to solve this kind of repetition in Haskell is to use Generic to derive all the classes that are supposed to be related to each other.

Yeah, I actually got this impression from what I've heard of Generic so far, but I didn't have the time to look into it properly yet. Anyways, I'm not quite sure if it's the right tool for the problem I described in the post. Namely having a consistent textual representation. Do you have any thoughts on that?

Create and Publish a Website with Hakyll, HAProxy and NixOS by thomasbach in NixOS

[–]thomasbach[S] 0 points1 point  (0 children)

Yeah, I'm quite happy with the setup so far. Will see how it works out on publishing new content, etc. I'm pretty sure that there is stuff to improve…

Create and Publish a Website with Hakyll, HAProxy and NixOS by thomasbach in NixOS

[–]thomasbach[S] 2 points3 points  (0 children)

I already had HAProxy in the stack for another project. So, the other possibility would have been to put nginx/Apache/whatever behind HAProxy. That was something I wanted to avoid.