Re: [Haskell-cafe] Binding of an integer variable
Another fun example: let c = 0 : c let b = 1 : 1 : c :show bindings length (take 5 c) :show bindings take 3 c :show bindings ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Binding of an integer variable
On Thu, 28 Jul 2011, Brandon Allbery wrote: On Thu, Jul 28, 2011 at 14:52, Henning Thielemann wrote: I have never used ':show bindings' before ... in other cases like let x='a' and let x=23::Int Since you gave explicit types, x is bound to strict values directly; it's only when no type is given that GHCi needs to translate the constant per the Language Report to an invocation of fromInteger. let x=23::Integer is left unevaluated, too. I do not understand, how fromInteger may change the state of evaluation. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Binding of an integer variable
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 : ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Binding of an integer variable
On Thu, Jul 28, 2011 at 14:52, Henning Thielemann < lemm...@henning-thielemann.de> wrote: > I have never used ':show bindings' before ... in other cases like > let x='a' > and > let x=23::Int > Since you gave explicit types, x is bound to strict values directly; it's only when no type is given that GHCi needs to translate the constant per the Language Report to an invocation of fromInteger. -- brandon s allbery allber...@gmail.com wandering unix systems administrator (available) (412) 475-9364 vm/sms ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Binding of an integer variable
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.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Binding of an integer variable
It hasn't been evaluated yet. It is just a thunk. >let x = 23 >:show bindings x :: Integer = _ >x 23 >:show bindings x :: Integer = 23 On Thu, Jul 28, 2011 at 2:43 PM, 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 > > > ___ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Binding of an integer variable
On Thu, Jul 28, 2011 at 14:43, 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? > 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. -- brandon s allbery allber...@gmail.com wandering unix systems administrator (available) (412) 475-9364 vm/sms ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
