I have a question about the following GHCi interaction:
Prelude> let x = 23
Prelude> :show bindings
x :: Integer = _
What is the meaning of the underscore in the third line? Why doesn't it say this, instead?
The underscore indicates an unevaluated thunk. In this case it's because numeric literals are translated into function calls (in this case (fromInteger 23); even though GHCi has already applied defaulting to the type, it still hasn't evaluated the value yet, and won't until it's needed due to laziness. If you do something like show x then the binding will be updated to show the actual value.