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

Maybe it would be better to introduce _ rather than using undefined for holes in programs. This sidesteps this issue, provides useful information to guide the implementation and causes an error when code is compiled so programs aren't unexpectedly partial. It is perhaps not idiomatic to use undefined, even in development these days. It is much easier (imo) to use holes and -fdefer-type-errors. 'undefined' is nasty and has to be used with care; CallStacks exist as a bit of a safety net. I've not read your book but I'm not convinced it should be emphasised so much in the first chapters of an elementary Haskell textbook. On Sat, Feb 13, 2016 at 8:52 PM, Christopher Allen wrote:

Replying to a few here.

Gigante:

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Just a question: how do you manage the type of simple arithmetic expressions like 1 + 1? I mean, the type contains a constraint there. Prelude> :t 1 + 1 1 + 1 :: Num a => a

Earlier versions of the book intentionally juked this or concreted the type of numbers to Integer, but HaskellForMac defenestrated that with the always-on type showing so now we've had an explanation along the lines you suggest for a few months now. We know how to adapt - we've tested the book extensively. What we need is the implementation not to expose irrelevant magic/noise in ways that could leak into the first chapter with Haskell code in it.

Seidel:

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If your readers are using :t they must already know about simple types like Integer, [], and, ->, so the new things are HasCallStack and =>.

You'd bloody think as we're careful when we introduce :t, but we get tickets from people using HaskellForMac[1] that get confused because they are shown the types of expressions too early. Now we're careful in how we introduce _any_ expressions.

Fisking your attempt:

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=> is just like -> except

They don't know what either of those things are or what they mean in the second chapter because this is the _first_ chapter with any Haskell code. They're just beginning to see how Haskell code might be kinda like the lambdas in the lambda calculus chapter.

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the compiler fills in the argument by

We do explain what a compiler and interpreter are, but they won't know what it means for it to fill in an argument. They don't know why it needs to fill in an argument. Where did the argument come from?

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HasCallStack tells the compiler

How? Why? Why do they need to care? What's a HasCallStack? Keep in mind they don't know types, typeclasses, or anything else.

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that the expression needs a call-stack

Still don't know what a call, stack, or call-stack are.

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because it might crash.

Why does that change the type? We can construct bottoms like `let x in x` that crash the program without changing the type.

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So HasCallStack => [Integer] is a [Integer]

What makes this even more obnoxious is that when we finally do introduce typeclasses and constraints, we talk about constraining a _type variable_ and now you've baked this magic in they cannot possibly be explained at all in the book.

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that might crash and produce a stack-trace

First bit they might pick up from context, they don't know what a stack trace is. Bonus round: when you explain things "from first principles", you can't duck the fact that it's actually a call graph when explaining a "stack" trace or call stack. Now you have to explain why/how it gets flattened from one representation into the other.

Oliver had it when he said,

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"What's a call stack?"

They don't know what a stack, a call, or the combination thereof is. We had planned to address these issues in the (much later) chapters that cover profiling and errors. Because that's when they're relevant. This hasn't been relevant the entire span of the book. It _never_ mattered that you didn't get a stack trace from bottoms. In practice, does it suck? Sure! But they're not practitioners yet! I am (I use Haskell for my 9-5 and have done for a year and a half) and it still hasn't mattered to me. The only time I've really wanted a stack trace is when this mechanism would not have been available to me to begin with.

Gamari / amindfv

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I don't have a copy of GHC 8 atm to test this with: is an expression like this now illegal?

x :: Int x = undefined

This is still valid. The change in GHC 8.0 is merely that GHC will infer a CallStack constraint instead of solving it in-place if asked to infer a type for a let binding whose RHS demands a callstack.

We have readers use the REPL _a lot_. Not only to load code but also free-standing expressions in the REPL when experimenting and learning. Type assignment in the REPL is noisy and we have to write around some pretty gnarly width limitations (40-60 cols). This breaks the examples where we're combining bottom and type inference to explore how terms and types interact.

I am less disturbed by `HasCallStack =>` than I was by the inferred type of ($).

I know designing around pedagogical limitations like this is tedious but imagine doing it for 900-1,200 pages (formatting varies) of tutorial and exercises, then getting unpleasant surprises right as the book is about to be done.

Sorry about the messy thread all.

[1]: http://haskellformac.com/

On Sat, Feb 13, 2016 at 1:48 PM, Bryan Richter wrote:

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On Sat, Feb 13, 2016 at 09:18:07AM -0800, Eric Seidel wrote:

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Here's what the GHCi session should look like.

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$ ghci GHCi, version 8.0.0.20160204: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/callen/.ghci Prelude> let myList = [1..5 :: Integer] Prelude> let myList' = myList ++ undefined Prelude> :t myList' myList' :: HasCallStack => [Integer]

What use case is satisfied by providing this information? How does it benefit the Haskell programmer? How do I use it?

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

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