[GHC] #14370: improve documentation of -fdefer-typed-holes for naked expressions in ghci (was: non-deferred typed hole despite -fdefer-typed-holes)

[GHC]

#14370: improve documentation of -fdefer-typed-holes for naked expressions in ghci
-------------------------------------+-------------------------------------
        Reporter:  int-e             |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Documentation     |              Version:  8.2.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  error/warning at compile-time      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by int-e):

 * type:  bug => task
 * component:  Compiler => Documentation


Old description:

> Consider the following ghci session.
>
> {{{
> GHCi, version 8.2.1: http://www.haskell.org/ghc/  :? for help
> Prelude> :set -fdefer-typed-holes -Wno-typed-holes
> Prelude> :set -fdefer-out-of-scope-variables -Wno-deferred-out-of-scope-
> variables
> Prelude> let x = [_]; y = [r]
> Prelude> (length x, length y)
> (1,1)
> Prelude> length [_]
>
> <interactive>:4:9: error:
>     • Found hole: _ :: a0
>       Where: ‘a0’ is an ambiguous type variable
>     • In the expression: _
>       In the first argument of ‘length’, namely ‘[_]’
>       In the expression: length [_]
>     • Relevant bindings include it :: Int (bound at <interactive>:4:1)
> Prelude> length [r]
> 1
> }}}
>
> Why does the `length [_]` expression produce a type error immediately
> instead of being deferred?
>
> (I've asked the same question in #14367 but this looks like a real bug.)

New description:

 Consider the following ghci session.

 {{{
 GHCi, version 8.2.1: http://www.haskell.org/ghc/  :? for help
 Prelude> :set -fdefer-typed-holes -Wno-typed-holes
 Prelude> :set -fdefer-out-of-scope-variables -Wno-deferred-out-of-scope-
 variables
 Prelude> let x = [_]; y = [r]
 Prelude> (length x, length y)
 (1,1)
 Prelude> length [_]

 <interactive>:4:9: error:
     • Found hole: _ :: a0
       Where: ‘a0’ is an ambiguous type variable
     • In the expression: _
       In the first argument of ‘length’, namely ‘[_]’
       In the expression: length [_]
     • Relevant bindings include it :: Int (bound at <interactive>:4:1)
 Prelude> length [r]
 1
 }}}

 Why does the `length [_]` expression produce a type error immediately
 instead of being deferred?

 ~~(I've asked the same question in #14367 but this looks like a real
 bug.)~~

 The documentation can be improved here, see [#comment:2 comment 2]

--

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14370#comment:4>
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