<html xmlns:o="urn:schemas-microsoft-com:office:office" xmlns:w="urn:schemas-microsoft-com:office:word" xmlns:m="http://schemas.microsoft.com/office/2004/12/omml" xmlns="http://www.w3.org/TR/REC-html40"><head><meta http-equiv=Content-Type content="text/html; charset=utf-8"><meta name=Generator content="Microsoft Word 15 (filtered medium)"><style><!--
/* Font Definitions */
@font-face
{font-family:"Cambria Math";
panose-1:2 4 5 3 5 4 6 3 2 4;}
@font-face
{font-family:Calibri;
panose-1:2 15 5 2 2 2 4 3 2 4;}
/* Style Definitions */
p.MsoNormal, li.MsoNormal, div.MsoNormal
{margin:0cm;
margin-bottom:.0001pt;
font-size:12.0pt;
font-family:"Calibri",sans-serif;}
a:link, span.MsoHyperlink
{mso-style-priority:99;
color:#0563C1;
text-decoration:underline;}
a:visited, span.MsoHyperlinkFollowed
{mso-style-priority:99;
color:#954F72;
text-decoration:underline;}
span.EmailStyle17
{mso-style-type:personal-compose;
font-family:"Calibri",sans-serif;}
.MsoChpDefault
{mso-style-type:export-only;
font-family:"Calibri",sans-serif;}
@page WordSection1
{size:612.0pt 792.0pt;
margin:72.0pt 72.0pt 72.0pt 72.0pt;}
div.WordSection1
{page:WordSection1;}
--></style></head><body lang=EN-CA link="#0563C1" vlink="#954F72"><div class=WordSection1><p class=MsoNormal><span style='font-size:11.0pt'>Hi ghc-devs,<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt'>I’m making a first attempt to make a frontend plugin, to resolve cabal packages in the GHC API. However I’m running into troubles with module resolution in the GHC API, because I can’t control where it will search for modules at all. I've attached a minimal example, with a frontend plugin definition that can’t find modules (Plugin.hs), and an equivalent standalone program that does (Main.hs). <o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt'>Specifically, I'm following a solution Edward Yang published in 2017 (http://blog.ezyang.com/2017/02/how-to-integrate-ghc-api-programs-with-cabal/), where the frontend plugin is called through a helper script that passes flags forwarded from `cabal repl`. To test the plugin directly with GHC, I collected the args through the helper script and filtered them to the minimal set that made the plugin run:<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt'> ghc --frontend Plugin -itarget -package-db dist-newstyle/packagedb/ghc-8.6.5 Plugin -plugin-package sandbox -hide-all-packages<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt'>This, as well as the full argument set, would complain that it can't find the target module under `./target/A.hs`:<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt'> <no location info>: error: module ‘A’ cannot be found locally<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt'>It does when the import path arg `-itarget` is absolute. Still, its `importPaths` are what I expect: [".", "target"], and the standalone program finds the target module with the same `importPaths`. I've tested this in GHC 8.6.5, 8.4.2 and 8.2.2, making me sure I'm just missing something, but I haven’t found help in the docs yet. I really appreciate some help to draw my hours over this to a close!<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt'>Thanks,<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt'>Derek<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt'>PS the full invocation of the plugin (with absolute paths swapped with relative ones, except `-itarget` which was relative to begin with) is:<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt'>ghc --frontend Plugin -plugin-package sandbox -fbuilding-cabal-package -O0 -outputdir dist-newstyle/build/x86_64-osx/ghc-8.6.5/sandbox-0.1.0.0/build -odir dist-newstyle/build/x86_64-osx/ghc-8.6.5/sandbox-0.1.0.0/build -hidir dist-newstyle/build/x86_64-osx/ghc-8.6.5/sandbox-0.1.0.0/build -stubdir dist-newstyle/build/x86_64-osx/ghc-8.6.5/sandbox-0.1.0.0/build -i -idist-newstyle/build/x86_64-osx/ghc-8.6.5/sandbox-0.1.0.0/build -i. -itarget -idist-newstyle/build/x86_64-osx/ghc-8.6.5/sandbox-0.1.0.0/build/autogen -idist-newstyle/build/x86_64-osx/ghc-8.6.5/sandbox-0.1.0.0/build/global-autogen -Idist-newstyle/build/x86_64-osx/ghc-8.6.5/sandbox-0.1.0.0/build/autogen -Idist-newstyle/build/x86_64-osx/ghc-8.6.5/sandbox-0.1.0.0/build/global-autogen -Idist-newstyle/build/x86_64-osx/ghc-8.6.5/sandbox-0.1.0.0/build -optP-include -optPdist-newstyle/build/x86_64-osx/ghc-8.6.5/sandbox-0.1.0.0/build/autogen/cabal_macros.h -this-unit-id sandbox-0.1.0.0-inplace -hide-all-packages -Wmissing-home-modules -no-user-package-db -package-db /Users/derek-lam/.cabal/store/ghc-8.6.5/package.db -package-db dist-newstyle/packagedb/ghc-8.6.5 -package-db dist-newstyle/build/x86_64-osx/ghc-8.6.5/sandbox-0.1.0.0/package.conf.inplace -package-id base-4.12.0.0 -package-id ghc-8.6.5 -XHaskell2010 Plugin -hide-all-packages<o:p></o:p></span></p></div></body></html>