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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Declaring TH dependency in `.cabal` file","articleBody":"Currently, happstack has\n\n```\nFlag template_haskell\n    Description: Template Haskell is available on this system\n    Default: True\n    Manual: True\n\n...\n\nLibrary:\n  if (flag(template_haskell) \u0026\u0026 !(arch(arm)))\n    Build-Depends:     template-haskell\n    cpp-options:       -DTEMPLATE_HASKELL\n\n...\n\n  Extensions: TemplateHaskell\n```\n\nThere's a couple of problems here (with cabal-1.24 \u0026 GHC 8.0)\n\nIf the TH-flag is set to false (or arch=arm), this package still won't build when GHC doesn't support TH, because then `extensions: TemplateHaskell` will still be in effect, and GHC will fail complaining it doesn't support TH.\n\nAnd with cabal-1.24, the solver will already exclude packages during solving where `extensions: TemplateHaskell` when GHC doesn't support that extension.\n\nMoreover, arch(arm) is not the only platform lacking TH support. However, since cabal knows when your environment supports TH, you should let cabal decide.\n\nConsequently, I suggest the following definitions instead: \n\nUse an automatic flag for declaring TH support, and make sure this flag doesn't have any other pressure applied other than `other-extensions: TemplateHaskell` or `default-extensions: TemplateHaskell`, i.e.\n\n```\nFlag template_haskell\n    Description: Template Haskell is available on this system\n    Default: True\n    Manual: False\n\nLibrary:\n  -- the TH library is available even in environments lacking -XTemplateHaskell support\n  -- placing this outside the conditional allows adding upper bounds after-the-fact on hackage w/o adding solver-pressure to the flag\n  Build-Depends: template-haskell\n  if flag(template_haskell)\n    cpp-options:       -DTEMPLATE_HASKELL\n    -- other-extensions doesn't enable the extension; this only declares which LANGUAGE pragmas may be used\n    other-extensions: TemplateHaskell\n```\n\nthis will cause cabal (starting with 1.24) to set the `template_haskell` flag  depending on whether the compiler supports TH.\n\n/cc @bergmark @phadej\n","author":{"url":"https://github.com/hvr","@type":"Person","name":"hvr"},"datePublished":"2016-06-10T11:56:41.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/28/happstack-server/issues/28"}