Why :sprint always prints a “_”?

Question

Prelude> let a = 3
Prelude> :sprint a
a = _
Prelude> let c = \"ab\"
Prelude> :sprint c
c = _

Why does it always print a _?

Answer 1

I'm a bit late, but I had a similar issue:

λ: let xs = [1,2,3]
xs :: Num t => [t]
λ: :sprint xs
xs = _
λ: print xs
λ: :sprint xs
xs = _

This issue is specific to polymorphic values. If you have -XNoMonomorphismRestriction enabled ghci will never really evaluate/force xs, it'll only evaluate/force specializations:

λ: :set -XMonomorphismRestriction
λ: let xs = [1,2,3]
xs :: [Integer]
λ: print xs
λ: :sprint xs
xs = [1,2,3]