On Thu, 28 Jul 2011, 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

I have never used ':show bindings' before ... in other cases like let x='a' and let x=23::Int the value of 'x' is shown. If 'x' is bound to a function, the underscore is shown. Thus I assume, that ':show bindings' uses the underscore for types without text representation. But it seems not to use the 'Show' class, since in this case Integers could be shown. The documentation does not mention the output of values, at all: http://www.haskell.org/ghc/docs/7.0-latest/html/users_guide/ghci-commands.ht...

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 : ....