`[source] <https://gitlab.haskell.org/ghc/ghc/tree/master/compiler/typecheck/TcClassDcl.hs>`_

compiler/typecheck/TcClassDcl.hs
================================


Note [Polymorphic methods]
~~~~~~~~~~~~~~~~~~~~~~~~~~

`[note link] <https://gitlab.haskell.org/ghc/ghc/tree/master/compiler/typecheck/TcClassDcl.hs#L390>`__

Consider
    class Foo a where
        op :: forall b. Ord b => a -> b -> b -> b
    instance Foo c => Foo [c] where
        op = e

When typechecking the binding 'op = e', we'll have a meth_id for op
whose type is
      op :: forall c. Foo c => forall b. Ord b => [c] -> b -> b -> b

So tcPolyBinds must be capable of dealing with nested polytypes;
and so it is. See TcBinds.tcMonoBinds (with type-sig case).



Note [Silly default-method bind]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

`[note link] <https://gitlab.haskell.org/ghc/ghc/tree/master/compiler/typecheck/TcClassDcl.hs#L405>`__

When we pass the default method binding to the type checker, it must
look like    op2 = e
not          $dmop2 = e
otherwise the "$dm" stuff comes out error messages.  But we want the
"$dm" to come out in the interface file.  So we typecheck the former,
and wrap it in a let, thus
          $dmop2 = let op2 = e in op2
This makes the error messages right.