Everything, and the same in reverse. That's the problem. Using haskell, I wish I was using go because the downsides of haskell bother me. But when I am using go, I wish I was using haskell because the downsides of go bother me.

Ah the quest for the perfect mix of everything. I sympathize but I've generally found that there are some things that are better solved in Haskell and others in Go and have found a generally pleasant balance.

I see. For me I haven't found anything like that, where task X would be better in go and task Y would be better in haskell. Everything I've done was always better in haskell. It was just an annoying pain in the ass the whole time because of super slow compile times, or running out of RAM compiling, or package conflicts from cabal hating me, or .cabal files requiring spaces instead of tabs, or haskell code I want to modify being some giant mess of arbitrarily aligned with a million spaces craziness that makes cool things like "diff" useless. It just feels like rather than "haskell needs to stop being so crazy", the "go needs to gain modern features" approach is more likely to actually be possible.

You can create a tool that lets you inline Haskell code in your Go sources. The resulting code could be compiled and called via C, some sort of IPC or even as a network service.