On Thursday 28 July 2011, 20:43:30, Paul Reiners wrote:
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?
x :: Integer = 23
Because x is not yet evaluated: Prelude> let x = 23 Prelude> :show bindings x :: Integer = _ Prelude> x 23 Prelude> :show bindings it :: Integer = 23 x :: Integer = 23 When bindings are shown, no evaluation takes place, things are only shown as far as they are evaluated, unevaluated parts are represented by an underscore, (partially) evaluated parts that aren't shown are represented by an ellipsis: Prelude> let xs = [1 .. 100] Prelude> :show bindings xs :: [Integer] = _ Prelude> head xs 1 Prelude> :show bindings it :: Integer = 1 xs :: [Integer] = 1 : _ Prelude> xs !! 3 4 Prelude> :show bindings it :: Integer = 4 xs :: [Integer] = 1 : 2 : 3 : 4 : _ Prelude> last xs 100 Prelude> :show bindings it :: Integer = 100 xs :: [Integer] = 1 : 2 : 3 : 4 : 5 : ....