Profiling executables which use Template Haskell fails

[snoyberg]

Repro:

$ cat EmbedString.hs 
module EmbedString (embedStr) where

import Language.Haskell.TH
import Language.Haskell.TH.Syntax (lift)

embedStr :: IO String -> ExpQ
embedStr readStr = lift =<< runIO readStr
$ cat Main.hs 
{-# LANGUAGE TemplateHaskell #-}

import EmbedString (embedStr)

main :: IO ()
main = putStr $(embedStr (readFile "Setup.hs"))
$ cat profth.cabal
cabal-version: 2.0
name:           profth
version:        0.1.0.0
build-type:     Simple

executable profth
  main-is: Main.hs
  other-modules: EmbedString
  build-depends: base, template-haskell
  default-language: Haskell2010
$ cabal new-build --enable-profiling
Resolving dependencies...
Build profile: -w ghc-8.6.4 -O1
In order, the following will be built (use -v for more details):
 - profth-0.1.0.0 (exe:profth) --enable-profiling (first run)
Configuring executable 'profth' for profth-0.1.0.0..
Preprocessing executable 'profth' for profth-0.1.0.0..
Building executable 'profth' for profth-0.1.0.0..
[1 of 2] Compiling EmbedString      ( EmbedString.hs, /Users/michael/Desktop/profth/dist-newstyle/build/x86_64-osx/ghc-8.6.4/profth-0.1.0.0/x/profth/build/profth/profth-tmp/EmbedString.p_o )
[2 of 2] Compiling Main             ( Main.hs, /Users/michael/Desktop/profth/dist-newstyle/build/x86_64-osx/ghc-8.6.4/profth-0.1.0.0/x/profth/build/profth/profth-tmp/Main.p_o )
Main.hs:6:15: fatal:
    cannot find object file ‘/Users/michael/Desktop/profth/dist-newstyle/build/x86_64-osx/ghc-8.6.4/profth-0.1.0.0/x/profth/build/profth/profth-tmp/EmbedString.dyn_o’
    while linking an interpreted expression
$ cabal --version
cabal-install version 2.4.1.0
compiled using version 2.4.1.0 of the Cabal library 
$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.6.4

[snoyberg]

And I just remembered that this is caused by a missing other-extensions: TemplateHaskell, which fixes this problem. It would be nice to get better documentation of this, but closing.

[flip111]

Maybe some message like "did you forget to put TemplateHaskell in other-extensions" would be nice when the fatal error occurs.

[23Skidoo]

Related: #4746, #1562.

[recursion-ninja]

Why do I need TemplateHaskell in the other-extensions of the .cabal file in order to build with profiling when my actual source files that use template Haskell enables TemplateHaskell via the language extension pragma?

I don't want TemplateHaskell enabled in every module of my project, only the modules that use it.
Having it enabled produces much, much worse parse error messages involving single quotes (').

[23Skidoo]

Cabal has no knowledge of LANGUAGE pragmas used by the package, and other-extensions (unlike default-extensions) doesn't enable the extension globally, it just informs Cabal that you used it somewhere.

[tomsmeding]

I just ran into this, trying to profile an executable with template haskell and getting the same fatal error as in the OP. The fix (adding other-extensions: TemplateHaskell) indeed works, but I would never have thought of this, because nothing in the documentation of other-extensions mentions anything related to changed semantics: it only says anything about checking GHC support, and even alludes to other-extensions being "informational". Hence, when I read the documentation for this cabal file key, I simply discarded it as not useful to me, because I use bounds on base to constrain GHC versions.

It would be great if not only the other-extensions documentation is updated to reflect its apparently fairly significant effect on what cabal does, and furthermore to detect the situation where the user specified --enable-profiling but is also using TemplateHaskell somehow. Because as a user, I have absolutely no clue how to go from "I'm using profiling and TemplateHaskell and GHC cannot find a .dyn_o file" to "let's use other-extensions", apart from using a search engine and hopefully finding this issue.