Commit 4a677f76 authored by Simon Marlow's avatar Simon Marlow
Browse files

Remove section about ApplicativeDo & existentials (#13875)

Summary:
This section is irrelevant now that strict pattern matches don't get
the ApplicativeDo treatment.

Test Plan:
```
make html FAST=YES
```

Reviewers: bgamari, austin, erikd

Subscribers: rwbarton, thomie

GHC Trac Issues: #13875

Differential Revision: https://phabricator.haskell.org/D4087
parent 2be55b85
......@@ -1074,46 +1074,6 @@ will always be connected with ``>>=``, to retain the same strictness
semantics as the standard do-notation. If you don't want this, simply
put a ``~`` on the pattern match to make it lazy.
 
.. _applicative-do-existential:
Existential patterns and GADTs
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When the pattern in a statement matches a constructor with
existential type variables and/or constraints, the transformation that
``ApplicativeDo`` performs may mean that the pattern does not scope
over the statements that follow it. This is because the rearrangement
happens before the expression is typechecked. For example, this
program does not typecheck::
{-# LANGUAGE RankNTypes, GADTs, ApplicativeDo #-}
data T where A :: forall a . Eq a => a -> T
test = do
A x <- undefined
_ <- return 'a'
_ <- return 'b'
return (x == x)
The reason is that the ``Eq`` constraint that would be brought into
scope from the pattern match ``A x`` is not available when
typechecking the expression ``x == x``, because ``ApplicativeDo`` has
rearranged the expression to look like this::
test =
(\x _ -> x == x)
<$> do A x <- undefined; _ <- return 'a'; return x
<*> return 'b'
(Note that the ``return 'a'`` and ``return 'b'`` statements are needed
to make ``ApplicativeDo`` apply despite the restriction noted in
:ref:`applicative-do-strict`, because ``A x`` is a strict pattern match.)
Turning off ``ApplicativeDo`` lets the program typecheck. This is
something to bear in mind when using ``ApplicativeDo`` in combination
with :ref:`existential-quantification` or :ref:`gadt`.
.. _applicative-do-pitfall:
 
Things to watch out for
......
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