gistfile1.txt

withCompiler :: Extension -> (forall i. Thrist (FromSExprShow :=> Compiler1) i X86 -> a) -> a
withCompiler ".L1" go = go (mapThrist Constrained l1Compiler)

executables/LC.hs:51:39:
    No instance for (FromSExpr y) arising from a use of ‘Constrained’
    Possible fix:
      add (FromSExpr y) to the context of
        a type expected by the context:
          Compiler1 x y -> (:=>) FromSExprShow Compiler1 x y
        or the type signature for
             withCompiler :: Extension
                             -> (forall i. Thrist (FromSExprShow :=> Compiler1) i X86 -> a) -> a
    In the first argument of ‘mapThrist’, namely ‘Constrained’
    In the first argument of ‘go’, namely
      ‘(mapThrist Constrained l1Compiler)’
    In the expression: go (mapThrist Constrained l1Compiler)

You need both the class and one instance, both with empty bodies; you probably need to add a ghc extension to define the instance.