Re: [Haskell-cafe] [] == []

GHC 7.8.3 here. Hmm, on second inspection I believe that the Bool reference is to the top-level expression, not the list items after all. I don't seem to get any defaulting behaviour either when not using GHCi. This seems to indicate that the extended interactive type defaulting is coming into play: https://downloads.haskell.org/~ghc/7.8.2/docs/html/users_guide/interactive-e... I'd guess that this means that the () default is being used. -Wall seems to indicate that this is in-fact the case: [Prelude] λ [] == [] <interactive>:2:4: Warning: Defaulting the following constraint(s) to type ‘()’ (Eq t0) arising from a use of ‘==’ In the expression: [] == [] In an equation for ‘it’: it = [] == [] True Hope this helps. - Lyndon On Tue, Mar 10, 2015 at 1:42 PM, Rustom Mody wrote:

On Tue, Mar 10, 2015 at 8:07 AM, Lyndon Maydwell wrote:

...

This looks like a great opportunity to try out the new holes functionality:

[Prelude] λ [] == _what

<interactive>:3:7: Found hole ‘_what’ with type: [t0] Where: ‘t0’ is an ambiguous type variable Relevant bindings include it :: Bool (bound at <interactive>:3:1) In the second argument of ‘(==)’, namely ‘_what’ In the expression: [] == _what In an equation for ‘it’: it = [] == _what

Looks like it defaults to [Bool]!

Correct me if I'm wrong :)

Thanks Lyndon. But on my (debian testing) ghc I get:

Prelude> [] == _what

<interactive>:2:7: Not in scope: `_what'

[ghc 7.6.3]

And my main question is the second one : For pedagogic purposes I want to get the error -- Whats the (family of) related options?