• Simon Peyton Jones's avatar
    A (final) re-engineering of the new typechecker · c80364f8
    Simon Peyton Jones authored
    Regression testing and user feedback for GHC 7.0 taught
    us a lot.  This patch fixes numerous small bugs, and some
    major ones (eg Trac #4484, #4492), and improves type
    error messages.
    The main changes are:
    * Entirely remove the "skolem equivalance class" stuff;
      a very useful simplification
    * Instead, when flattening "wanted" constraints we generate
      unification variables (not flatten-skolems) for the
      flattened type function application
    * We then need a fixup pass at the end, TcSimplify.solveCTyFunEqs,
      which resolves any residual equalities of form
          F xi ~ alpha
    * When we come across a definite failure (e.g. Int ~ [a]),
      we now defer reporting the error until the end, in case we
      learn more about 'a'.  That is particularly important for
      occurs-check errors.  These are called "frozen" type errors.
    * Other improvements in error message generation.
    * Better tracing messages