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

Haskell is lazy. It doesn't evaluate things until they are needed.

The GHCi sprint command (not part of Haskell, just a debugging command of the interpreter) prints the value of an expression without forcing evaluation.

When you write

let a = 3

you bind a new name a to the right-hand side expression, but Haskell won't evaluate that thing yet. Therefore, when you sprint it, it prints _ as the value to indicate that the expression has not yet been evaluated.

Try this:

let a = 3
:sprint a -- a has not been evaluated yet
print a -- forces evaluation of a
:sprint a -- now a has been evaluated