`Reference to undefined compilation unit` in utop

Learning
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1

In the past, I have been able to install a package using opam install ... and then access it from inside utop using like #require "...".

For the past few days, I’ve been having an issue with this. For example, I just installed timedesc by running opam install timedesc and when I use #require "timedesc" in utop, I get the following response:

Error: Reference to undefined compilation unit `Timedesc_tzlocal'
Hint: This means that the interface of a module is loaded, but its implementation is not.
      Did you mean to load a compiled implementation of the module
      using #load or by passing it as an argument to the toplevel?

I am a little puzzled about what this means, and what to do about it. Can anyone help? Thanks!

2

I believe this happens whenever a library depends on another library that makes use of dune’s “virtual libraries”. For example, I see the same behaviour with Irmin in the top-level:

Error: Reference to undefined compilation unit `Digestif'
Hint: This means that the interface of a module is loaded, but its implementation is not.
      Did you mean to load a compiled implementation of the module
      using #load or by passing it as an argument to the toplevel?

Whatever way the top-level works means you have to decide on a concrete implementation of the interface provided by the dependency. In the case of timedesc this looks like:

utop # #require "timedesc-tzlocal.unix";;
utop # #require "timedesc-tzdb.full";;
utop # #require "timedesc";;

From a dune perspective, a given library can program purely against the interface, whereas an executable must decide upon the implementation. Hope this helps, I’m sure someone can explain the exact details a lot better!

1 Like
3

Thanks, this was really helpful!