Re: [Haskell-cafe] Printing the empty list.
On 01.07.2011 00:58, Joshua Ball wrote: GHCi seems to be clever about some things: If I try to print the empty list in ghci, I encounter no problems: Prelude> [] [] Prelude> show [] "[]" Prelude> print [] [] Even though the type of the list is clearly unknown, it must be picking SOME type. (why does it print [] instead of "")? If I write a program in a file and load it in main = print [] Then I get the ambiguous type variable error that I would expect. Why doesn't ghci generate this error at the prompt? GHCi will warn you, that type defaulting was used, if you start it with ghci -Wall ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Printing the empty list.
Figuring out how to tell what type ghci is defaulting to was an interesting exercise. The sum [] trick seemed cool, so I tried a variant: Prelude> let f xs = const xs $ show xs Prelude> f [] [] Prelude> :t it it :: [()] -- ryan On Thu, Jun 30, 2011 at 6:44 PM, Ivan Lazar Miljenovic < ivan.miljeno...@gmail.com> wrote: > On 1 July 2011 11:35, Brent Yorgey wrote: > > On Fri, Jul 01, 2011 at 09:05:05AM +1000, Ivan Lazar Miljenovic wrote: > >> On 1 July 2011 08:58, Joshua Ball wrote: > >> > GHCi seems to be clever about some things: > >> > > >> > If I try to print the empty list in ghci, I encounter no problems: > >> > > >> > Prelude> [] > >> > [] > >> > Prelude> show [] > >> > "[]" > >> > Prelude> print [] > >> > [] > >> > > >> > Even though the type of the list is clearly unknown, it must be > >> > picking SOME type. (why does it print [] instead of "")? > >> > >> Type defaulting: if you don't specify a type, then ghci makes it > >> [Integer]. > > > > In this case I'm pretty sure it is [()] since there is only a Show > > constraint. If there were a Num constraint it would pick > > Integer. > > Yeah, I forgot about () > > -- > Ivan Lazar Miljenovic > ivan.miljeno...@gmail.com > IvanMiljenovic.wordpress.com > > ___ > 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] Printing the empty list.
On 1 July 2011 11:35, Brent Yorgey wrote: > On Fri, Jul 01, 2011 at 09:05:05AM +1000, Ivan Lazar Miljenovic wrote: >> On 1 July 2011 08:58, Joshua Ball wrote: >> > GHCi seems to be clever about some things: >> > >> > If I try to print the empty list in ghci, I encounter no problems: >> > >> > Prelude> [] >> > [] >> > Prelude> show [] >> > "[]" >> > Prelude> print [] >> > [] >> > >> > Even though the type of the list is clearly unknown, it must be >> > picking SOME type. (why does it print [] instead of "")? >> >> Type defaulting: if you don't specify a type, then ghci makes it >> [Integer]. > > In this case I'm pretty sure it is [()] since there is only a Show > constraint. If there were a Num constraint it would pick > Integer. Yeah, I forgot about () -- Ivan Lazar Miljenovic ivan.miljeno...@gmail.com IvanMiljenovic.wordpress.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Printing the empty list.
On Fri, Jul 01, 2011 at 09:05:05AM +1000, Ivan Lazar Miljenovic wrote: > On 1 July 2011 08:58, Joshua Ball wrote: > > GHCi seems to be clever about some things: > > > > If I try to print the empty list in ghci, I encounter no problems: > > > > Prelude> [] > > [] > > Prelude> show [] > > "[]" > > Prelude> print [] > > [] > > > > Even though the type of the list is clearly unknown, it must be > > picking SOME type. (why does it print [] instead of "")? > > Type defaulting: if you don't specify a type, then ghci makes it > [Integer]. In this case I'm pretty sure it is [()] since there is only a Show constraint. If there were a Num constraint it would pick Integer. Prelude> sum [] 0 Prelude> :t it it :: Integer -Brent ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Printing the empty list.
On 1 July 2011 08:58, Joshua Ball wrote: > GHCi seems to be clever about some things: > > If I try to print the empty list in ghci, I encounter no problems: > > Prelude> [] > [] > Prelude> show [] > "[]" > Prelude> print [] > [] > > Even though the type of the list is clearly unknown, it must be > picking SOME type. (why does it print [] instead of "")? Type defaulting: if you don't specify a type, then ghci makes it [Integer]. > If I write a program in a file and load it in > > main = print [] > > Then I get the ambiguous type variable error that I would expect. Why > doesn't ghci generate this error at the prompt? Because ghc doesn't do type defaulting. -- Ivan Lazar Miljenovic ivan.miljeno...@gmail.com IvanMiljenovic.wordpress.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Printing the empty list.
On Thu, Jun 30, 2011 at 18:58, Joshua Ball wrote: > GHCi seems to be clever about some things: GHCi uses extended defaulting rules unless told otherwise, so in the absence of anything else it uses () as the type. You can enable this in GHC as well, with -XExtendedDefaultRules. See http://www.haskell.org/ghc/docs/latest/html/users_guide/interactive-evaluation.html#extended-default-rules -- 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
[Haskell-cafe] Printing the empty list.
GHCi seems to be clever about some things: If I try to print the empty list in ghci, I encounter no problems: Prelude> [] [] Prelude> show [] "[]" Prelude> print [] [] Even though the type of the list is clearly unknown, it must be picking SOME type. (why does it print [] instead of "")? If I write a program in a file and load it in main = print [] Then I get the ambiguous type variable error that I would expect. Why doesn't ghci generate this error at the prompt? -- Borrow my books: http://goo.gl/UBbSH ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
