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

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.miljenovic@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.Miljenovic@gmail.com IvanMiljenovic.wordpress.com

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