package problem with "res"

[David Eisenbud]

Surely "res" (or resolution or freeResolution) is the most basic M2 command. But recently when I install or load packages, M2 complains that its 

 

stdio:1:16:(3): error: mutable unexported unset symbol(s) in package ResBug: 'res'
ResBug.m2:15:14-15:17: here is the first use of 'res'

Example below. How should this be handled?

--

David Eisenbud

Professor of Mathematics

University of California Berkeley

https://eisenbud.github.io/

 newPackage(
         "ResBug",
         Version => "0.1",
         Date => "August 30, 2025",
         Headline => "problem with unexported res",
         Authors => {{ Name => "DE"}},
         DebuggingMode => true
         )

     export {"resFormat"}
     -* Code section *-

     resFormat = method()
     resFormat Module := M -> (
F := res M;
for i from 0 to length F list rank F_i)
     
     -* Documentation section *-
     beginDocumentation()

     doc ///
     Key
      ResBug
     Headline
      problem with "unexported" res
     Description
       Text
        How can this be fixed?
       Example
        S = ZZ/101[x,y]

M = coker vars SresFormat M

     
///
     -* Test section *-
     
     end--
     restart
     loadPackage "ResBug"
     restart
     installPackage "ResBug"     

[David Eisenbud]

I pushed return and sent my previous message prematurely. The Dod section in my email has bad formatting,

but I believe that the formatting is correct in my emacs file, attached.

--

David Eisenbud

Professor of Mathematics

University of California Berkeley

https://eisenbud.github.io/

[Doug Torrance]

The issue is that "res" is no longer exported by Core -- it's exported by whichever homological algebra package the user has loaded (the new Complexes or the old OldChainComplexes).

So one fix would be to add the HomologicalAlgebraPackage variable (whose value is the user's preferred complexes package) to PackageExports or PackageImports, e.g.,:

newPackage(...

    PackageExports => {HomologicalAlgebraPackage}

)

[David Eisenbud]

Thanks, Doug! -- David

--

David Eisenbud

Professor of Mathematics

University of California Berkeley

https://eisenbud.github.io/

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