April 2016 - ghc-devs

Odd error message due to PolyKinds
by Kosyrev Serge 10 Apr '16

10 Apr '16

Good day folks, Here is a test case that exposes a certain weakness in PolyKinds error messaging. In the following snippet (T W) :: * is malkinded: > {-# LANGUAGE PolyKinds #-} > {-# LANGUAGE UnicodeSyntax #-} > > class Eq a ⇒ EqSyn a where > data T w > data W f > > x = (undefined ∷ EqSyn w ⇒ T w → ()) (undefined ∷ T W) 8.0.1rc2 yields: > ../ghc-playground.hs:8:42: error: > • Expected kind ‘T W’, but ‘undefined :: T W’ has kind ‘T W’ > • In the first argument of ‘undefined :: > EqSyn w => T w -> ()’, namely > ‘(undefined :: T W)’ > In the expression: > (undefined :: EqSyn w => T w -> ()) (undefined :: T W) > In an equation for ‘main’: > main = (undefined :: EqSyn w => T w -> ()) (undefined :: T W) ..which is.. not informative. With PolyKinds removed: > ../ghc-playground.hs:7:56: error: > • Expecting one more argument to ‘W’ > Expected a type, but ‘W’ has kind ‘* -> *’ > • In the first argument of ‘T’, namely ‘W’ > In an expression type signature: T W > In the first argument of ‘undefined :: > EqSyn w => T w -> ()’, namely > ‘(undefined :: T W)’ I'm not sure if this was reported, so I'm sorry for the noise if it was. -- с уважениeм / respectfully, Косырев Сергей

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Benchmarking harnesses for a more modern nofib?
by Ryan Newton 07 Apr '16

07 Apr '16

Hi all, Is anyone currently working in, or interested in helping with, a new benchmark suite for Haskell? Perhaps, packaging up existing apps and app benchmarks into a new benchmark suite that gives a broad picture of Haskell application performance today? Background: We run nofib, and we run the shootout benchmarks. But when we want to evaluate basic changes to GHC optimizations or data representation, these really don't give us a full picture of whether a change is beneficial. A few years ago, fibon <https://hackage.haskell.org/package/fibon> tried to gather some Hackage benchmarks. This may work even better with Stackage, where there are 180 benchmark suites among the 1770 packages currently. Also, these days companies are building substantial apps in Hackage. Which substantial apps could or should go into a benchmark suite? I see Warp and other web server benchmarks <http://www.infoq.com/news/2015/04/web-frameworks-benchmark-2015?utm_source=…> all over the web. But is there a harness that can time some of this code while running inside a single-machine, easy-setup benchmark suite? Best, -Ryan

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Trouble with first GHC patch submission: uncommitted changes
by Conal Elliott 06 Apr '16

06 Apr '16

I'm trying to get through my first GHC patch submission, using git and Phabricator, following directions at https://ghc.haskell.org/trac/ghc/wiki/Phabricator. "git status" shows two unstaged changes: "deleted: test spaces" and "modified: ../libraries/bytestring (modified content)" (in a submodule). I don't know where either change came from. When I use "arc diff" to submit my intended change to the compiler, I get asked about these to uncommitted changes. I say to ignore the submodule change and do not amend my commit with the "test spaces" deletion, I get "Usage Exception: You can not continue with uncommitted changes. Commit or discard them before proceeding." I don't know how to discard (nor whether safe), and I don't think I want to commit. Note: the directions say to use "arc diff~" (rather than "arc diff"), to which arc replied "(Assuming 'diff~' is the British spelling of 'diff'.)". Thanks for any help! -- Conal

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"opt_univ fell into a hole"
by Conal Elliott 06 Apr '16

06 Apr '16

I'm getting the following error message from a GHC plugin I'm developing (note GHC version): ghc-stage2: panic! (the 'impossible' happened) (GHC version 8.1.20160307 for x86_64-apple-darwin): opt_univ fell into a hole {aD1S} I don't get this error when compiling without my plugin, so I may well be violating a compiler invariant. Shortly before the error, the plugin produced the following dictionary expression, which does indeed contain a `UnivCo` with a `HoleProv`. The plugin does sometimes generate `UnivCo`s but not with `HoleProv`. let { $dHasRep_aD1T :: HasRep (Vec 'Z s_aD1R[fuv:0]) $dHasRep_aD1T = $fHasRepVec0 @ s_aD1R[fuv:0] } in let { $dHasRepZLVeczqZZZLKeyhZRZR_Ii5CR :: HasRep (Vec 'Z (Key h_aCnh)) $dHasRepZLVeczqZZZLKeyhZRZR_Ii5CR = $dHasRep_aD1T `cast` ((HasRep (Vec <'Z>_N (Sym U(hole:{aD1S}, Key h_aCnh, s_aD1R[fuv:0])_N))_N)_R :: HasRep (Vec 'Z s_aD1R[fuv:0]) ~R# HasRep (Vec 'Z (Key h_aCnh))) } in $dHasRepZLVeczqZZZLKeyhZRZR_Ii5CR Here, `Key` is a type family from `Data.Key` in the keys package, and `Vec` is a GADT of statically length-indexed lists. Suggestions? Thanks, - Conal

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Feature development
by Sazanovich Nikita 06 Apr '16

06 Apr '16

Hello! I am working on ticket https://ghc.haskell.org/trac/ghc/ticket/5850 <https://ghc.haskell.org/trac/ghc/ticket/5850> now. I’ve come upon converting the type, which GHC.compileExpr returns, to my prompt function type [PModule] -> Int -> IO String. But this function returns ghc-prim-0.5.0.0:GHC.Prim.Any. I don’t see a clear way to convert it. So my questions are: - What this type is about? I know Any type in Haskell. Is it a specific version of it? - Or maybe it is not the best way to check a function correctness? I’ve checked out InteractiveEval module and it seems that there are only four quite similar functions to compile Expressions. Last person, who was trying to solve this problem, used Unsafe.Coerce and it is also doing this job pretty well now, but the name speaks for itself. Thank you.

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whats the current state of runtime rep for ghc 8 onwards?
by Carter Schonwald 05 Apr '16

05 Apr '16

i was reading https://ghc.haskell.org/trac/ghc/wiki/NoSubKinds and looking at some of the applicable code, and it looks like i can't quantify over ptr-y heap values that may or may not be lifted, at least with the current state of play, is this correct?

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Initial compile time benchmarks
by Ömer Sinan Ağacan 01 Apr '16

01 Apr '16

Hi all, Using Ben's timing patch [^1], Cabal, and a Haskell program to parse generated logs [^2], I generated some tables that show compile times of modules in hxt, haskell-src-exts, lens, and all of their dependencies: https://gist.githubusercontent.com/osa1/fd82335181a584679c9f3200b7b0a8a5/ra… Some notes: - Modules and passes in modules are sorted. - At the end of the file you can see the cumulative numbers for the passes. In fact, that's a small table so I'm just going to paste it here: ======================Total====================== CodeGen 41.32% Simplifier 34.83% Renamer/typechecker 12.22% Desugar 2.11% CorePrep 1.90% Demand analysis 1.44% CoreTidy 1.35% Called arity analysis 1.10% Float inwards 0.96% Common sub-expression 0.87% Parser 0.75% SpecConstr 0.57% Specialise 0.30% Worker Wrapper binds 0.17% Liberate case 0.12% ByteCodeGen 0.00% Simplify 0.00% I don't know how to make use of this yet, but I thought ghc-devs may find it useful. As a next thing I'm hoping to add some more `withTiming` calls. The analyze program can handle nested `withTiming` calls and renders passes as a tree (GHC HEAD doesn't have nested `withTiming`s so we don't see it in the file above), so we can benchmark things in more details. I also want to experiment a little bit with different `force` parameters to `withTiming`. If anyone has any other ideas I can also try those. --- [^1]: https://phabricator.haskell.org/D1959 [^2]: https://github.com/osa1/analyze-ghc-timings

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