Is there a way to make GHC provide the type class constraints of typed holes?

Question

Current behavior

Prelude> show _  <interactive>:7:6:     Found hole ‘_’ with type: a0     Where: ‘a0’ is an ambiguous type variable     Relevant bindings include it :: String (bound at <interactive>:7:1)     In the first argument of ‘show’, namely ‘_’     In the expression: show _     In an equation for ‘it’: it = show _ 

Desired behavior

It would be nice if GHC would also tell me that the typed hole has the Show type class constraint.

Misc

GHC Version 7.8.1

Answer 1

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) 

Answer 2

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