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

I've put together a patch to prevent GHC from inferring CallStacks for *top-level* expressions. Given that we've hidden the implicit parameter behind a type synonym (and may remove it entirely in the future), I'm less concerned about preserving the expected behavior for implicit parameters. I believe this should address most of your concerns wrt ghci interactions. ghci> let myList = [1, 2, 3 :: Integer] myList :: [Integer] ghci> let myList' = myList ++ undefined myList' :: [Integer] ghci> :t myList myList :: [Integer] ghci> :t myList' myList' :: [Integer] ghci> :sprint myList' myList' = _ ghci> head myList' 1 it :: Integer ghci> :sprint myList' myList' = 1 : _ Note that undefined still takes a CallStack, and thus has a more involved type. ghci> :t undefined undefined :: forall (v :: GHC.Types.Levity) (a :: TYPE v). GHC.Stack.Types.HasCallStack => a But uses of undefined will no longer infect the top-level context with CallStacks, you'll have to request them explicitly. (We need the inference for local binders, so HaskellForMac and ghc-mod could still show CallStacks in tooltips). The patch is awaiting review at https://phabricator.haskell.org/D1912, you're more than welcome to comment. Eric On Sat, Feb 13, 2016, at 16:49, Christopher Allen wrote:

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What I'd suggest instead is to define your own undefined in a Prelude-replacement (or simplification if you will).

No. Part of the reason for the book is so that people can learn in the environment that they'll use and be equipped to apply what they've learned with minimal surprises. This solution is worse than the others suggested so far. And _again_, it's not just about our book it's about learning resources more generally and what that experience is like for new people.

Rust doesn't need to have a beginner's Prelude. Idris doesn't either. In fact, most languages don't and the only one that has pulled it off convincingly is Racket which had less cause to do so than GHC does at this juncture.

On Sat, Feb 13, 2016 at 6:02 PM, Eric Seidel wrote:

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On Sat, Feb 13, 2016, at 15:32, Christopher Allen wrote:

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There's another problem I discovered with HasCallStack. The implicit parameter constraint breaks sharing behavior for examples that use bottom.

This is a necessary consequence of the implementation of callstack-aware functions as overloaded functions. It's really no different from using a type-class, and is in my opinion the correct behavior.

It seems what you really want is a way to prevent GHC from inferring the HasCallStack constraint in the first place. That's doable with an explicit type signature (or even a combinator to wrap the expression), but this isn't a great solution for your book.

What I'd suggest instead is to define your own undefined in a Prelude-replacement (or simplification if you will). Something like

undefined :: a undefined = withFrozenCallStack emptyCallStack Prelude.undefined

should work to remove all traces of CallStacks (including when it blows up). error can be similarly wrapped to avoid having to deal with CallStacks, and $ and the FTP-related functions can be specialized to less-polymorphic versions that are easier to explain to beginners (at least until they're ready to be exposed to the real versions).

I'm sorry that this change has caused you trouble. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

-- Chris Allen Currently working on http://haskellbook.com