How to check from the REPL that a given constraint is satisfied?

For the sake of exploring libraries with complex types and typeclass instances, I want to check from the REPL whether a typeclass constraint is satisfied or not (provided all the relevant dataypes and instances are in scope).

For example: in rel8 1.5.0, is the constraint Table Expr (Expr Int, Expr Bool) satisfied? (It's not, as Int lacks a DBType instance, so the preconditions fail).

Non-solutions:

Using :kind!. This only checks that the constraint is well-formed, not that it's satisified.

ghci> :kind! (Table Expr (Expr Int, Expr Bool))
(Table Expr (Expr Int, Expr Bool)) :: Constraint
= Table Expr (Expr Int, Expr Bool)

Using :instances. Doesn't seem to work with multi-parameter typeclasses like Table, and moreover it returns all possible instances, which can be verbose.

ghci> :instances Expr
instance Reifiable Expr
instance Nullifiable Expr
instance GRecordable Expr

ghci> :instances (Expr Int, Expr Bool)
instance [safe] NotNull (Expr Int, Expr Bool)
instance [safe] Nullable (Expr Int, Expr Bool)
instance Show (Expr Int, Expr Bool) 
instance GHC.Generics.Generic (Expr Int, Expr Bool)

Solution

I have defined the following function in my .ghci file:

:set -XAllowAmbiguousTypes
:{
yep :: forall c . c => ()
yep = ()
:}

Which can be used like this

ghci> yep @(Table Expr (Expr Int, Expr Bool))
<interactive>:4:1: error: [GHC-39999]
    • No instance for ‘DBType Int’ arising from a use of ‘yep’
    • In the expression: yep @(Table Expr (Expr Int, Expr Bool))
      In an equation for ‘it’:
          it = yep @(Table Expr (Expr Int, Expr Bool))

ghci> yep @(Table Expr (Expr Int32, Expr Bool))
()