No currently its not possible.But it may be added to GHC as per the speculations.

This is now fixed in GHC 8.0 thanks to @DominiqueDevriese's GHC ticket.

Due to extended type defaulting, this isn't immediately obvious in GHCi. With your example,

> show _


<interactive>:7:6: error:
• Found hole: _h :: ()
Or perhaps ‘_h’ is mis-spelled, or not in scope
• In the first argument of ‘show’, namely ‘_h’
In the expression: show _h
In an equation for ‘it’: it = show _h
• Relevant bindings include
it :: String (bound at <interactive>:7:1)

the type of the hole is defaulted to (). This is apparently the desired behavior, though there's an argument to be made that extended defaulting shouldn't apply to holes (as a common use for them is to get the compiler to tell you the inferred type).

Nevertheless, if you compile with GHC or disable extended default rules in GHCi (via :set -XNoExtendedDefaultRules), we see the result of the improvements:

<interactive>:3:1: error:
• Ambiguous type variable ‘a0’ arising from a use of ‘show’
prevents the constraint ‘(Show a0)’ from being solved.
Probable fix: use a type annotation to specify what ‘a0’ should be.
These potential instances exist:
instance Show Ordering -- Defined in ‘GHC.Show’
instance Show Integer -- Defined in ‘GHC.Show’
instance Show a => Show (Maybe a) -- Defined in ‘GHC.Show’
...plus 22 others
...plus 11 instances involving out-of-scope types
(use -fprint-potential-instances to see them all)
• In the expression: show _
In an equation for ‘it’: it = show _


<interactive>:3:6: error:
• Found hole: _ :: a0
Where: ‘a0’ is an ambiguous type variable
• In the first argument of ‘show’, namely ‘_’
In the expression: show _
In an equation for ‘it’: it = show _
• Relevant bindings include
it :: String (bound at <interactive>:3:1)

Try it :: _ => _ in GHC 8.8+.