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I just downloaded a fresh installation of the Haskell platform on Mac El Capitàn:

    brew cask install haskell-platform

and a fresh installation of the Haskell layer for spacemacs:

   dotspacemacs-configuration-layers
   '( ;;; lots of other layers
     haskell)

However, I get errors on almost every keystroke in emacs

Try installing a more recent version of haskell-stack-ghc, and please open a bug report if the issue persists in the latest release.  Thanks!
Suspicious state from syntax checker haskell-stack-ghc: Flycheck checker haskell-stack-ghc returned non-zero exit code 1, but its output contained no errors: Downloading lts-10.0 build plan ...
Downloaded lts-10.0 build plan.
AesonException "Error in $.packages.cassava.constraints.flags['bytestring--lt-0_10_4']: Invalid flag name: \"bytestring--lt-0_10_4\""

This results in very slow operation as it (apparently) redownloads lts-10.0 build plan on almost every keystroke.

The rest of the interface to the ghci interpreter seems to work. The defect only affects my editing in Haskell mode.

I'd be grateful even for hacky workarounds not including just disabling Haskell-mode.

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Turns out this was a conflict between haskell-platform and haskell-stack. Cured by the manual download option here

https://docs.haskellstack.org/en/stable/install_and_upgrade/

The homebrew version did not work.

Very painful to figure this out and debug it (only figured it out by trying to install the Haskell extensions for IntelliJ, exposing the conflict). I am sure I don't understand the issues in any depth, but I got it working.

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    Mark your answer as an solution if it solved your problem. – user12563 Jan 26 '18 at 19:21

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