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Trying to determine when to insert parentheses during TH
conversion is a bit of a mess. There is an assortment of functions
that try to detect this, such as:
* `hsExprNeedsParens`
* `isCompoundHsType`
* `hsPatNeedsParens`
* `isCompoundPat`
* etc.
To make things worse, each of them have slightly different semantics.
Plus, they don't work well in the presence of explicit type
signatures, as #14875 demonstrates.
All of these problems can be alleviated with the use of an explicit
precedence argument (much like what `showsPrec` currently does). To
accomplish this, I introduce a new `PprPrec` data type, and define
standard predences for things like function application, infix
operators, function arrows, and explicit type signatures (that last
one is new). I then added `PprPrec` arguments to the various
`-NeedsParens` functions, and use them to make smarter decisions
about when things need to be parenthesized.
A nice side effect is that functions like `isCompoundHsType` are
now completely unneeded, since they're simply aliases for
`hsTypeNeedsParens appPrec`. As a result, I did a bit of refactoring
to remove these sorts of functions. I also did a pass over various
utility functions in GHC for constructing AST forms and used more
appropriate precedences where convenient.
Along the way, I also ripped out the existing `TyPrec`
data type (which was tailor-made for pretty-printing `Type`s) and
replaced it with `PprPrec` for consistency.
Test Plan: make test TEST=T14875
Reviewers: alanz, goldfire, bgamari
Reviewed By: bgamari
Subscribers: rwbarton, thomie, carter
GHC Trac Issues: #14875
Differential Revision: https://phabricator.haskell.org/D4688
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