improve documentation of -fdefer-typed-holes for naked expressions in ghci
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 (closed) but this looks like a real bug.)~~
The documentation can be improved here, see [#ticket:14370#comment:144137 comment 2]