[Haskell-cafe] New type of expressions containing (error ...) includes noisy implicit parameter

Eric Seidel eric at seidel.io
Sat Feb 13 17:00:08 UTC 2016

On Sat, Feb 13, 2016, at 01:04, Oliver Charles wrote:
> Just a clarification - the presence of that parameter does not say "I use
> call stack" but rather "I need *a* call stack". This subtle difference
> means that you do need to know about this due to how stacks work in GHC.
> If
> you use error and then don't also add that constraint yourself to your
> function, youll only see part of the stack. Hence it more than just UX
> noise - I have a decision to make, and building a call stack gas a cost.
> Also, is this RC2? I thought this was now changed to use constraint kinds
> and type alias the implicit parameter behind a nicer API.

Yes, RC2 still infers implicit parameter constraints as opposed to
HasCallStack constraints. This is a bug, and it should be a simple fix.


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