I’ve been happily using Haskell for a couple of years through Nix like so:

(pkgs.haskell.packages.ghc910.override {overrides = ...;}).ghcWithPackages(p: [...])

However recently one of my haskell apps started crashing with an exception from time to time. I am hunting for a way to have a call-stack printed for these exceptions. Since I don’t control third party libraries I cannot add HasCallstack to them to be able to debug that way so I have been trying to compile with -prof for the last couple of days with no success.

Does anyone have a working example of how to get this working?

In reply to @iclanzan:matrix.org

I’ve been happily using Haskell for a couple of years through Nix like so:

(pkgs.haskell.packages.ghc910.override {overrides = ...;}).ghcWithPackages(p: [...])

However recently one of my haskell apps started crashing with an exception from time to time. I am hunting for a way to have a call-stack printed for these exceptions. Since I don’t control third party libraries I cannot add HasCallstack to them to be able to debug that way so I have been trying to compile with -prof for the last couple of days with no success.

Does anyone have a working example of how to get this working?

If you really need to, you can override dependencies to tune them to your liking.

But profiling builds are available by default, so if that's all you're after then you only need to tweak the build for the final executable.

What exactly have you tried and how has it failed? The usual Cabal flag should just work.

I am not using Cabal in my project. Trying to compile with ghc -prof gives an error about ghc not supporting profiling because it was compiled with -dynamic. I tried overriding mkDerivation to add enableLibraryProfiling = true and enableExecutableProfiling = true which caused the tests (check phase) to fail for a lot of libraries so then I added doCheck = false to skip tests. That worked to compile libraries but did not help with ghc. Same error ensued.

Then I tried overriding pkgs.haskell.packages.ghc910.compiler by adding enableProfiledLibs = true to no effect. I then added enableSharedLibraries = false and enableSharedExecutables = false and that cause ghc to fail to compile complaining about missing dynamic object files.

Works fine for me.

alex@alex-pc3 ~> nix-shell -p ghc --run fish
alex@alex-pc3 nix-shell:/t/nix-shell-1930075-2124826253> ghc -prof Hello.hs
[1 of 2] Compiling Main             ( Hello.hs, Hello.o )
[2 of 2] Linking Hello
alex@alex-pc3 nix-shell:/t/nix-shell-1930075-2124826253> ./Hello -RTS -p
Hello, world!
alex@alex-pc3 nix-shell:/t/nix-shell-1930075-2124826253> cat Hello.prof
	Mon Mar  2 21:25 2026 Time and Allocation Profiling Report  (Final)

	   Hello +RTS -p -s -RTS

	total time  =        0.00 secs   (0 ticks @ 1000 us, 1 processor)
	total alloc =      49,280 bytes  (excludes profiling overheads)

COST CENTRE MODULE                    SRC              %time %alloc

MAIN        MAIN                      <built-in>         0.0   22.7
CAF         GHC.Internal.IO.Handle.FD <entire-module>    0.0   70.5
CAF         GHC.Internal.IO.Encoding  <entire-module>    0.0    5.0
CAF         GHC.Internal.Conc.Signal  <entire-module>    0.0    1.3


                                                                               individual      inherited
COST CENTRE  MODULE                         SRC             no.     entries  %time %alloc   %time %alloc

MAIN         MAIN                           <built-in>      138           0    0.0   22.7     0.0  100.0
 CAF         GHC.Internal.Conc.Signal       <entire-module> 262           0    0.0    1.3     0.0    1.3
 CAF         GHC.Internal.IO.Encoding       <entire-module> 208           0    0.0    5.0     0.0    5.0
 CAF         GHC.Internal.IO.Encoding.Iconv <entire-module> 206           0    0.0    0.4     0.0    0.4
 CAF         GHC.Internal.IO.Handle.FD      <entire-module> 197           0    0.0   70.5     0.0   70.5
 CAF         Main                           <entire-module> 145           0    0.0    0.1     0.0    0.1

For reproducibility, I should also mention this:

alex@alex-pc3 ~> nixos-version
25.11.2793.89dbf01df72e (Xantusia)

Works fine for me.

alex@alex-pc3 ~> nix-shell -p ghc --run fish
alex@alex-pc3 nix-shell:/t/nix-shell-1930075-2124826253> ghc -prof Hello.hs
[1 of 2] Compiling Main             ( Hello.hs, Hello.o )
[2 of 2] Linking Hello
alex@alex-pc3 nix-shell:/t/nix-shell-1930075-2124826253> ./Hello +RTS -p
Hello, world!
alex@alex-pc3 nix-shell:/t/nix-shell-1930075-2124826253> cat Hello.prof
	Mon Mar  2 21:25 2026 Time and Allocation Profiling Report  (Final)

	   Hello +RTS -p -RTS

	total time  =        0.00 secs   (0 ticks @ 1000 us, 1 processor)
	total alloc =      49,280 bytes  (excludes profiling overheads)

COST CENTRE MODULE                    SRC              %time %alloc

MAIN        MAIN                      <built-in>         0.0   22.7
CAF         GHC.Internal.IO.Handle.FD <entire-module>    0.0   70.5
CAF         GHC.Internal.IO.Encoding  <entire-module>    0.0    5.0
CAF         GHC.Internal.Conc.Signal  <entire-module>    0.0    1.3


                                                                               individual      inherited
COST CENTRE  MODULE                         SRC             no.     entries  %time %alloc   %time %alloc

MAIN         MAIN                           <built-in>      138           0    0.0   22.7     0.0  100.0
 CAF         GHC.Internal.Conc.Signal       <entire-module> 262           0    0.0    1.3     0.0    1.3
 CAF         GHC.Internal.IO.Encoding       <entire-module> 208           0    0.0    5.0     0.0    5.0
 CAF         GHC.Internal.IO.Encoding.Iconv <entire-module> 206           0    0.0    0.4     0.0    0.4
 CAF         GHC.Internal.IO.Handle.FD      <entire-module> 197           0    0.0   70.5     0.0   70.5
 CAF         Main                           <entire-module> 145           0    0.0    0.1     0.0    0.1