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

compiler/deSugar/TmOracle.hs
============================


Note [Deep equalities]
~~~~~~~~~~~~~~~~~~~~~~

`[note link] <https://gitlab.haskell.org/ghc/ghc/tree/master/compiler/deSugar/TmOracle.hs#L251>`__

Solving nested equalities is the most difficult part. The general strategy
is the following:

  * Equalities of the form (True ~ (e1 ~ e2)) are transformed to just
    (e1 ~ e2) and then treated recursively.

  * Equalities of the form (False ~ (e1 ~ e2)) cannot be analyzed unless
    we know more about the inner equality (e1 ~ e2). That's exactly what
    `simplifyEqExpr' tries to do: It takes e1 and e2 and either returns
    truePmExpr, falsePmExpr or (e1' ~ e2') in case it is uncertain. Note
    that it is not e but rather e', since it may perform some
    simplifications deeper.