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should TcCanonical.rewriteEvidence preserve ctev_nosh?
by Nicolas Frisby 05 Jun '19

05 Jun '19

A bit of context and then my question. https://gitlab.haskell.org/ghc/ghc/blob/master/compiler/typecheck/TcCanonic… declares `rewriteEvidence` with the comment that "rewriteEvidence old_ev new_pred co ... Returns a new_ev : new_pred, with same wanted/given/derived flag as old_ev". However, the `CtWanted` case just immediately calls `newWanted`, which ignores the incoming `ctev_nosh` flag and always uses `WDeriv`. ``` rewriteEvidence ev@(CtWanted { ctev_dest = dest , ctev_loc = loc }) new_pred co = do { mb_new_ev <- newWanted loc new_pred ``` (This is true in 8.6.5 and also right now at 6febc444c0abea6c033174aa0e813c950b9b2877.) Thus I'm seeing a [W] is become a [WD], which seems to contradict the comment's "same flag" claim. MY QUESTION ======> Is this "WD unsplitting" the intended behavior? See canonicalization tc-trace below. If it is intended, I'll probably write a follow-up question asking for more advice. The constraint solver is looping unproductively because my plugin keeps discarding the (re-)emitted [D] shadow as a tautology, then the [W] "unsplits" during canonicalization (even though my plugin didn't touch the [W]), this re-emits the same [D] (via a fundep), and so on. My continual disclaimer recently is that I'm seeing this behavior with my in-development plugin activated, so it's entirely I'm pushing GHC into unfamiliar waters or even just plain violating some invariants without realizing it. But this trace looks rather isolated from plugins. Thanks! -Nick Here's a -ddump-tc-trace of a canonicalization that goes from [W] to [WD] happen (with GHC 8.6.5): runStage canonicalization { workitem = [W] $dMonadFree_a35V {0}:: MonadFree ((fs_a2UF[sk:1] :- Const e_a2UE[sk:1]) :+ Const e_a2UE[sk:1]) (Free fs_a2UF[sk:1]) (CDictCan) flatten_args { (fs_a2UF[sk:1] :- Const e_a2UE[sk:1]) :+ Const e_a2UE[sk:1] Free fs_a2UF[sk:1] matchFamTcM Matching: (fs_a2UF[sk:1] :- Const e_a2UE[sk:1]) :+ Const e_a2UE[sk:1] Match failed matchFamTcM Matching: fs_a2UF[sk:1] :- Const e_a2UE[sk:1] Match failed Unfilled tyvar fs_a2UF[sk:1] Unfilled tyvar e_a2UE[sk:1] flatten/flat-cache hit :- [* -> *, fs_a2UF[sk:1], Const e_a2UE[sk:1]] s_a35Z[fmv:1] Unfilled tyvar s_a35Z[fmv:1] Unfilled tyvar e_a2UE[sk:1] matchFamTcM Matching: s_a35Z[fmv:1] :+ Const e_a2UE[sk:1] Match failed New coercion hole: co_a365 Emitting new coercion hole {co_a365} :: (s_a35Z[fmv:1] :+ Const e_a2UE[sk:1]) GHC.Prim.~# s_a364[fmv:1] extendFlatCache :+ [* -> *, s_a35Z[fmv:1], Const e_a2UE[sk:1]] [WD] s_a364[fmv:1] flatten/flat-cache miss :+ [* -> *, s_a35Z[fmv:1], Const e_a2UE[sk:1]] s_a364[fmv:1] [WD] hole{co_a365} {0}:: (s_a35Z[fmv:1] :+ Const e_a2UE[sk:1]) GHC.Prim.~# s_a364[fmv:1] Unfilled tyvar fs_a2UF[sk:1] flatten } s_a364[fmv:1] Free fs_a2UF[sk:1] Emitting new wanted $dMonadFree_a366 :: MonadFree s_a364[fmv:1] (Free fs_a2UF[sk:1]) arising from a use of ‘wrap’ at tests/ill-typed/T7.hs:77:14-17 addTcEvBind a2V1 [W] $dMonadFree_a35V = $dMonadFree_a366 `cast` ((MonadFree (Sym {co_a365} ; ((:+) <* -> *>_N (Sym {co_a360}) <Const e_a2UE[sk:1]>_N)_N) <Free fs_a2UF[sk:1]>_N)_R :: MonadFree s_a364[fmv:1] (Free fs_a2UF[sk:1]) ~R# MonadFree ((fs_a2UF[sk:1] :- Const e_a2UE[sk:1]) :+ Const e_a2UE[sk:1]) (Free fs_a2UF[sk:1])) canClass [W] $dMonadFree_a35V {0}:: MonadFree ((fs_a2UF[sk:1] :- Const e_a2UE[sk:1]) :+ Const e_a2UE[sk:1]) (Free fs_a2UF[sk:1]) MonadFree s_a364[fmv:1] (Free fs_a2UF[sk:1]) ContinueWith [WD] $dMonadFree_a366 {0}:: MonadFree s_a364[fmv:1] (Free fs_a2UF[sk:1]) end stage canonicalization } and later try_fundeps [WD] $dMonadFree_a36h {0}:: MonadFree s_a36f[fmv:1] (Free fs_a2UF[sk:1]) (CDictCan) emitFunDepDeriveds 1 0 [fs_a2UF[sk:1] ~ s_a36f[fmv:1]] False Emitting new derived equality [D] _ {0}:: fs_a2UF[sk:1] GHC.Prim.~# s_a36f[fmv:1] and then my plugin discards that [D] as a tautological, and then the [W] gets unsplit and the [D] comes back around etc

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Wiki markup
by Simon Peyton Jones 04 Jun '19

04 Jun '19

This is a GitLab wiki markup question. Suppose I want to have a bulleted list, in which a single item has some text, then some laid-out code, then more text. I want the second blob of text to be indented as part of the bullet. Absent the laid-out code, indenting the second para 2 spaces seems to do it: * first bit of text Second bit of text But if there is code in the middle, like this * first bit of text ``` code ``` Second bit of text the second bit of text is not indented. Does anyone know how to achieve the indentation? Simon

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Re: Container type classes
by Andreas Klebinger 04 Jun '19

04 Jun '19

ghc-devs-request(a)haskell.org schrieb: > Hello, > > I think refactoring to use consistent naming is a good idea, but I am > not sure about the class idea. > > To see if it is viable, we should list the types in question and the > operations we'd like to overload. > > I find that with containers there tend to be two cases: either the > operations are similar but not exactly the same and you have to do > type hackery to make things fit, or you realize that you can just use > the same type in multiple places. > > Iavor The function prototype are already part of the merge request. See here: https://gitlab.haskell.org/ghc/ghc/blob/a0781d746c223636a90a0837fe678aab5b7… As for the data structures in question these are: * EnumSet * Data.IntSet * Data.Set * UniqSet * UniqDSet * Data.IntMap * Data.Map * LabelMap * UniqFM * UniqDFM * UniqMap * Maybe the TrieMap Variants Maybe I missed some but these are all I can think of currently. But they are already plenty. Imo using type classes IS a kind of type hackery required "to make things fit".

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RE: Pushing to gitlab.haskell.org
by Simon Peyton Jones 03 Jun '19

03 Jun '19

| You probably need to use the "git@" remote. | | git@gitlab.haskell.org:ghc/ghc.git | | git remote set-url origin git@gitlab.haskell.org:ghc/ghc.git Correct! I don't know how this worked before. (Maybe it never did; this is a relatively new machine.) Maybe we should change the guidance on https://gitlab.haskell.org/ghc/ghc/wikis/building/getting-the-sources to give the git url? Or does that /require/ ssh and hence exclude some folk? Simon | -----Original Message----- | From: Matthew Pickering <matthewtpickering(a)gmail.com> | Sent: 03 June 2019 15:08 | To: Simon Peyton Jones <simonpj(a)microsoft.com> | Subject: Re: Pushing to gitlab.haskell.org | | You probably need to use the "git@" remote. | | git@gitlab.haskell.org:ghc/ghc.git | | git remote set-url origin git@gitlab.haskell.org:ghc/ghc.git | | | On Mon, Jun 3, 2019 at 3:04 PM Simon Peyton Jones via ghc-devs <ghc- | devs(a)haskell.org> wrote: | > | > Devs | > | > Suddenly I can’t push to (a non-master branch on) gitlab.haskell.org. | > | > In .git/config I have | > | > [remote "origin"] | > | > url = | > https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitl | > ab.haskell.org%2Fghc%2Fghc&data=02%7C01%7Csimonpj%40microsoft.com% | > 7C186b15a68a134060a1bd08d6e82ce8b3%7C72f988bf86f141af91ab2d7cd011db47% | > 7C1%7C0%7C636951676910548930&sdata=cTjUrROo3Dje%2FlbiFdtUPadlFUPsN | > 2OWUgLIOYv%2BL8o%3D&reserved=0 | > | > fetch = +refs/heads/*:refs/remotes/origin/* | > | > [branch "master"] | > | > remote = origin | > | > merge = refs/heads/master | > | > An attempt to push leads to a request for a username. If I enter one, a | request for a password. | > | > simonpj@MSRC-3645512:~/code/HEAD$ git push origin wip/T16735 | > | > Username for | 'https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab. | haskell.org&data=02%7C01%7Csimonpj%40microsoft.com%7C186b15a68a134060a | 1bd08d6e82ce8b3%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C6369516769105 | 48930&sdata=O%2F%2B7WovumSLxe8%2FXrh1NpowWs%2FYVqhdEsDCsM5diz%2Bk%3D&a | mp;reserved=0': C-c C-c | > | > I thought I had SSH keys uploaded. But in fact I found that my | id_rsa.pub was a bit different to the one in gitlab. Not sure _how_ that | happened, or why it hasn’t bitten before. | > | > So I uploaded a new id_rsa.pub key. | > | > Same behaviour. | > | > So I tried ssh -v gitlab.haskell.org, and got the output below. It | seems to skip id_dsa, but doesn’t even try id_rsa. | > | > Can anyone help? This is tiresome. | > | > Thanks | > | > Simon | > | > | > | > simonpj@MSRC-3645512:~/code/HEAD$ ssh -v gitlab.haskell.org | > | > OpenSSH_7.6p1 Ubuntu-4, OpenSSL 1.0.2n 7 Dec 2017 | > | > debug1: Reading configuration data /home/simonpj/.ssh/config | > | > debug1: Reading configuration data /etc/ssh/ssh_config | > | > debug1: /etc/ssh/ssh_config line 19: Applying options for * | > | > debug1: Connecting to gitlab.haskell.org [2604:1380:0:8900::3] port 22. | > | > debug1: Connection established. | > | > debug1: identity file /home/simonpj/.ssh/id_rsa type 0 | > | > debug1: key_load_public: No such file or directory | > | > debug1: identity file /home/simonpj/.ssh/id_rsa-cert type -1 | > | > debug1: identity file /home/simonpj/.ssh/id_dsa type 1 | > | > debug1: key_load_public: No such file or directory | > | > debug1: identity file /home/simonpj/.ssh/id_dsa-cert type -1 | > | > debug1: key_load_public: No such file or directory | > | > debug1: identity file /home/simonpj/.ssh/id_ecdsa type -1 | > | > debug1: key_load_public: No such file or directory | > | > debug1: identity file /home/simonpj/.ssh/id_ecdsa-cert type -1 | > | > debug1: key_load_public: No such file or directory | > | > debug1: identity file /home/simonpj/.ssh/id_ed25519 type -1 | > | > debug1: key_load_public: No such file or directory | > | > debug1: identity file /home/simonpj/.ssh/id_ed25519-cert type -1 | > | > debug1: Local version string SSH-2.0-OpenSSH_7.6p1 Ubuntu-4 | > | > debug1: Remote protocol version 2.0, remote software version | > OpenSSH_7.9 | > | > debug1: match: OpenSSH_7.9 pat OpenSSH* compat 0x04000000 | > | > debug1: Authenticating to gitlab.haskell.org:22 as 'simonpj' | > | > debug1: SSH2_MSG_KEXINIT sent | > | > debug1: SSH2_MSG_KEXINIT received | > | > debug1: kex: algorithm: curve25519-sha256(a)libssh.org | > | > debug1: kex: host key algorithm: ssh-ed25519 | > | > debug1: kex: server->client cipher: chacha20-poly1305(a)openssh.com MAC: | > <implicit> compression: none | > | > debug1: kex: client->server cipher: chacha20-poly1305(a)openssh.com MAC: | > <implicit> compression: none | > | > debug1: expecting SSH2_MSG_KEX_ECDH_REPLY | > | > debug1: Server host key: ssh-ed25519 | > SHA256:wtE9yoEHLsotzqKfbDs1JIqR+c8ZZyW+FAJAmteBfcI | > | > debug1: Host 'gitlab.haskell.org' is known and matches the ED25519 host | key. | > | > debug1: Found key in /home/simonpj/.ssh/known_hosts:7 | > | > debug1: rekey after 134217728 blocks | > | > debug1: SSH2_MSG_NEWKEYS sent | > | > debug1: expecting SSH2_MSG_NEWKEYS | > | > debug1: SSH2_MSG_NEWKEYS received | > | > debug1: rekey after 134217728 blocks | > | > debug1: Skipping ssh-dss key /home/simonpj/.ssh/id_dsa - not in | > PubkeyAcceptedKeyTypes | > | > debug1: SSH2_MSG_EXT_INFO received | > | > debug1: kex_input_ext_info: | > server-sig-algs=<ssh-ed25519,ssh-rsa,rsa-sha2-256,rsa-sha2-512,ssh-dss | > ,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521> | > | > debug1: SSH2_MSG_SERVICE_ACCEPT received | > | > debug1: Authentications that can continue: | > publickey,password,keyboard-interactive | > | > debug1: Next authentication method: publickey | > | > debug1: Offering public key: RSA | > SHA256:H1l824hIxHozwRuEqdaYJjW10mRRlQVUhxREvuseqtU | > /home/simonpj/.ssh/id_rsa | > | > debug1: Authentications that can continue: | > publickey,password,keyboard-interactive | > | > debug1: Trying private key: /home/simonpj/.ssh/id_ecdsa | > | > debug1: Trying private key: /home/simonpj/.ssh/id_ed25519 | > | > debug1: Next authentication method: keyboard-interactive | > | > Password: C-c C-c | > | > | > | > simonpj@MSRC-3645512:~/code/HEAD$ | > | > | > | > _______________________________________________ | > ghc-devs mailing list | > ghc-devs(a)haskell.org | > https://nam06.safelinks.protection.outlook.com/?url=http%3A%2F%2Fmail. | > haskell.org%2Fcgi-bin%2Fmailman%2Flistinfo%2Fghc-devs&data=02%7C01 | > %7Csimonpj%40microsoft.com%7C186b15a68a134060a1bd08d6e82ce8b3%7C72f988 | > bf86f141af91ab2d7cd011db47%7C1%7C0%7C636951676910548930&sdata=d3AF | > 6HM%2B48XW8mZJ42m%2FSngTPiVEERDM24th%2BZMY%2BPA%3D&reserved=0

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setsockopt IPV2_TCLASS 8
by Simon Peyton Jones 03 Jun '19

03 Jun '19

I'm getting a lot of these messages from ssh when using git setsockopt IPV6_TCLASS 8: Operation not permitted: (Same thing when directly invoking ssh git(a)gitlab.haskell.org<mailto:git@gitlab.haskell.org>) Should I worry? Simon

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Pushing to gitlab.haskell.org
by Simon Peyton Jones 03 Jun '19

03 Jun '19

Devs Suddenly I can't push to (a non-master branch on) gitlab.haskell.org. In .git/config I have [remote "origin"] url = https://gitlab.haskell.org/ghc/ghc fetch = +refs/heads/*:refs/remotes/origin/* [branch "master"] remote = origin merge = refs/heads/master An attempt to push leads to a request for a username. If I enter one, a request for a password. simonpj@MSRC-3645512:~/code/HEAD$ git push origin wip/T16735 Username for 'https://gitlab.haskell.org': C-c C-c I thought I had SSH keys uploaded. But in fact I found that my id_rsa.pub was a bit different to the one in gitlab. Not sure _how_ that happened, or why it hasn't bitten before. So I uploaded a new id_rsa.pub key. Same behaviour. So I tried ssh -v gitlab.haskell.org, and got the output below. It seems to skip id_dsa, but doesn't even try id_rsa. Can anyone help? This is tiresome. Thanks Simon simonpj@MSRC-3645512:~/code/HEAD$ ssh -v gitlab.haskell.org OpenSSH_7.6p1 Ubuntu-4, OpenSSL 1.0.2n 7 Dec 2017 debug1: Reading configuration data /home/simonpj/.ssh/config debug1: Reading configuration data /etc/ssh/ssh_config debug1: /etc/ssh/ssh_config line 19: Applying options for * debug1: Connecting to gitlab.haskell.org [2604:1380:0:8900::3] port 22. debug1: Connection established. debug1: identity file /home/simonpj/.ssh/id_rsa type 0 debug1: key_load_public: No such file or directory debug1: identity file /home/simonpj/.ssh/id_rsa-cert type -1 debug1: identity file /home/simonpj/.ssh/id_dsa type 1 debug1: key_load_public: No such file or directory debug1: identity file /home/simonpj/.ssh/id_dsa-cert type -1 debug1: key_load_public: No such file or directory debug1: identity file /home/simonpj/.ssh/id_ecdsa type -1 debug1: key_load_public: No such file or directory debug1: identity file /home/simonpj/.ssh/id_ecdsa-cert type -1 debug1: key_load_public: No such file or directory debug1: identity file /home/simonpj/.ssh/id_ed25519 type -1 debug1: key_load_public: No such file or directory debug1: identity file /home/simonpj/.ssh/id_ed25519-cert type -1 debug1: Local version string SSH-2.0-OpenSSH_7.6p1 Ubuntu-4 debug1: Remote protocol version 2.0, remote software version OpenSSH_7.9 debug1: match: OpenSSH_7.9 pat OpenSSH* compat 0x04000000 debug1: Authenticating to gitlab.haskell.org:22 as 'simonpj' debug1: SSH2_MSG_KEXINIT sent debug1: SSH2_MSG_KEXINIT received debug1: kex: algorithm: curve25519-sha256(a)libssh.org debug1: kex: host key algorithm: ssh-ed25519 debug1: kex: server->client cipher: chacha20-poly1305(a)openssh.com MAC: <implicit> compression: none debug1: kex: client->server cipher: chacha20-poly1305(a)openssh.com MAC: <implicit> compression: none debug1: expecting SSH2_MSG_KEX_ECDH_REPLY debug1: Server host key: ssh-ed25519 SHA256:wtE9yoEHLsotzqKfbDs1JIqR+c8ZZyW+FAJAmteBfcI debug1: Host 'gitlab.haskell.org' is known and matches the ED25519 host key. debug1: Found key in /home/simonpj/.ssh/known_hosts:7 debug1: rekey after 134217728 blocks debug1: SSH2_MSG_NEWKEYS sent debug1: expecting SSH2_MSG_NEWKEYS debug1: SSH2_MSG_NEWKEYS received debug1: rekey after 134217728 blocks debug1: Skipping ssh-dss key /home/simonpj/.ssh/id_dsa - not in PubkeyAcceptedKeyTypes debug1: SSH2_MSG_EXT_INFO received debug1: kex_input_ext_info: server-sig-algs=<ssh-ed25519,ssh-rsa,rsa-sha2-256,rsa-sha2-512,ssh-dss,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521> debug1: SSH2_MSG_SERVICE_ACCEPT received debug1: Authentications that can continue: publickey,password,keyboard-interactive debug1: Next authentication method: publickey debug1: Offering public key: RSA SHA256:H1l824hIxHozwRuEqdaYJjW10mRRlQVUhxREvuseqtU /home/simonpj/.ssh/id_rsa debug1: Authentications that can continue: publickey,password,keyboard-interactive debug1: Trying private key: /home/simonpj/.ssh/id_ecdsa debug1: Trying private key: /home/simonpj/.ssh/id_ed25519 debug1: Next authentication method: keyboard-interactive Password: C-c C-c simonpj@MSRC-3645512:~/code/HEAD$

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Cmm syntax page on wiki
by Takenobu Tani 01 Jun '19

01 Jun '19

Hi devs, I wrote a wiki page for Cmm syntax [1] to improve readability for ghc developers. Could you tell me if there are any problems or errors? I'll update them or remove the page. [1]: https://gitlab.haskell.org/ghc/ghc/wikis/commentary/compiler/cmm-syntax Regards, Takenobu

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search on GItlab
by Simon Peyton Jones 31 May '19

31 May '19

If I search in GitLab for "checkExpectedKind" I don't fine #16635, which clearly mentions that. Why not? Thanks Simon

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RE: Container type classes
by Andrey Mokhov 31 May '19

31 May '19

Thanks again Iavor, Despite the type inference issue, and the fact that this requires a separate type class, this is the best solution I've seen so far. Cheers, Andrey -----Original Message----- From: Iavor Diatchki [mailto:iavor.diatchki@gmail.com] Sent: 30 May 2019 23:16 To: Andrey Mokhov <andrey.mokhov(a)newcastle.ac.uk> Cc: Brandon Allbery <allbery.b(a)gmail.com>; Andreas Klebinger <klebinger.andreas(a)gmx.at>; ghc-devs(a)haskell.org Subject: Re: Container type classes Yeah, there is really no relation between the two parameters of `Fun`, so you'd have to specify the intermediate type manually. For example: add3 :: forall s. (Fun s s, Elem s ~ Int) => s -> s add3 = colMap @s (+1) . colMap (+2) I wouldn't say that it's a particularly convenient interface to work with, unless you are working in a setting where most of the containers have known types. On Thu, May 30, 2019 at 2:58 PM Andrey Mokhov <andrey.mokhov(a)newcastle.ac.uk> wrote: > > Many thanks Iavor, > > This looks very promising! I played with your encoding a little, but quickly came across type inference issues. The following doesn't compile: > > add3 :: (Fun s s, Elem s ~ Int) => s -> s > add3 = colMap (+1) . colMap (+2) > > I'm getting: > > * Could not deduce: Elem a0 ~ Int > from the context: (Fun s s, Elem s ~ Int) > bound by the type signature for: > add3 :: forall s. (Fun s s, Elem s ~ Int) => s -> s > Expected type: Elem a0 -> Elem s > Actual type: Int -> Int > The type variable `a0' is ambiguous > > Fun s s is supposed to say that the intermediate type is `s` too, but I guess this is not how type class resolution works. > > Cheers, > Andrey > > -----Original Message----- > From: Iavor Diatchki [mailto:iavor.diatchki@gmail.com] > Sent: 30 May 2019 22:38 > To: Brandon Allbery <allbery.b(a)gmail.com> > Cc: Andrey Mokhov <andrey.mokhov(a)newcastle.ac.uk>; Andreas Klebinger <klebinger.andreas(a)gmx.at>; ghc-devs(a)haskell.org > Subject: Re: Container type classes > > This is how you could define `map`. This is just for fun, and to > discuss Haskell idioms---I am not suggesting we should do it. Of > course, it might be a bit more general than what you'd like---for > example it allows defining instances like `Fun IntSet (Set Int)` that, > perhaps?, you'd like to disallow: > > {-# LANGUAGE MultiParamTypeClasses, TypeFamilies #-} > > import Data.Set (Set) > import qualified Data.Set as Set > import Data.IntSet (IntSet) > import qualified Data.IntSet as ISet > > class Col t where > type Elem t > -- ... As in Andreas's example > > class (Col a, Col b) => Fun a b where > colMap :: (Elem a -> Elem b) -> a -> b > > instance Col (Set a) where > type Elem (Set a) = a > > instance Col IntSet where > type Elem IntSet = Int > > instance Fun IntSet IntSet where > colMap = ISet.map > > instance Ord b => Fun (Set a) (Set b) where > colMap = Set.map > > On Thu, May 30, 2019 at 2:32 PM Brandon Allbery <allbery.b(a)gmail.com> wrote: > > > > They can, with more work. You want indexed monads, so you can describe types that have e.g. an ordering constraint as well as the Monad constraint. > > > > On Thu, May 30, 2019 at 5:26 PM Andrey Mokhov <andrey.mokhov(a)newcastle.ac.uk> wrote: > >> > >> Hi Artem, > >> > >> > >> > >> Thanks for the pointer, but this doesn’t seem to be a solution to my challenge: they simply give up on overloading `map` for both Set and IntSet. As a result, we can’t write polymorphic functions over Set and IntSet if they involve any mapping. > >> > >> > >> > >> I looked at the prototype by Andreas Klebinger, and it doesn’t include the method `setMap` either. > >> > >> > >> > >> Perhaps, Haskell’s type classes just can’t cope with this problem. > >> > >> > >> > >> *ducks for cover* > >> > >> > >> > >> Cheers, > >> > >> Andrey > >> > >> > >> > >> From: Artem Pelenitsyn [mailto:a.pelenitsyn@gmail.com] > >> Sent: 30 May 2019 20:56 > >> To: Andrey Mokhov <andrey.mokhov(a)newcastle.ac.uk> > >> Cc: ghc-devs(a)haskell.org; Andreas Klebinger <klebinger.andreas(a)gmx.at> > >> Subject: Re: Container type classes > >> > >> > >> > >> Hi Andrey, > >> > >> > >> > >> FWIW, mono-traversable (http://hackage.haskell.org/package/mono-traversable) suggests decoupling IsSet and Funtor-like. > >> > >> > >> > >> In a nutshell, they define the IsSet class (in Data.Containers) with typical set operations like member and singleton, union and intersection. And then they tackle a (seemingly) independent problem of mapping monomorphic containers (like IntSet, ByteString, etc.) with a separate class MonoFunctor (in Data.MonoTraversable): > >> > >> > >> > >> class MonoFunctor mono where > >> omap :: (Element mono -> Element mono) -> mono -> mono > >> > >> > >> > >> And gazillion of instances for both polymorphic containers with a fixed type parameter and monomorphic ones. > >> > >> > >> > >> -- > >> > >> Best wishes, > >> > >> Artem > >> > >> > >> > >> On Thu, 30 May 2019 at 20:11, Andrey Mokhov <andrey.mokhov(a)newcastle.ac.uk> wrote: > >> > >> Hi all, > >> > >> I tried to use type classes for unifying APIs of several similar data structures and it didn't work well. (In my case I was working with graphs, instead of sets or maps.) > >> > >> First, you rarely want to be polymorphic over the set representation, because you care about performance. You really want to use that Very.Special.Set.insert because it has the right performance characteristics for your task at hand. I found only *one* use-case for writing polymorphic functions operating on something like IsSet: the testsuite. Of course, it is very nice to write a single property test like > >> > >> memberInsertProperty x set = (member x (insert x set) == True) > >> > >> and then use it for testing all set data structures that implement `member` and `insert`. Here you don't care about performance, only about correctness! > >> > >> However, this approach leads to problems with type inference, confusing error messages, and complexity. I found that it is much nicer to use explicit dictionary passing and write something like this instead: > >> > >> memberInsertProperty SetAPI{..} x set = (member x (insert x set) == True) > >> > >> where `member` and `insert` come from the SetAPI record via RecordWildCards. > >> > >> Finally, I'm not even sure how to create a type class covering Set and IntSet with the following two methods: > >> > >> singleton :: a -> Set a > >> map :: Ord b => (a -> b) -> Set a -> Set b > >> > >> singleton :: Int -> IntSet > >> map :: (Int -> Int) -> IntSet -> IntSet > >> > >> Could anyone please enlighten me about the right way to abstract over this using type classes? > >> > >> I tried a few approaches, for example: > >> > >> class IsSet s where > >> type Elem s > >> singleton :: Elem s -> s > >> map :: Ord (Elem t) => (Elem s -> Elem t) -> s -> t > >> > >> Looks nice, but I can't define the IntSet instance: > >> > >> instance IsSet IntSet where > >> type Elem IntSet = Int > >> singleton = IntSet.singleton > >> map = IntSet.map > >> > >> This fails with: Couldn't match type `t' with `IntSet' -- and indeed, how do I tell the compiler that in the IntSet case s ~ t in the map signature? Shall I add more associated types, or "associated constraints" using ConstraintKinds? I tried and failed, at various stages, repeatedly. > >> > >> ...And then you discover that there is Set.cartesianProduct :: Set a -> Set b -> Set (a, b), but no equivalent in IntSet and things get even more grim. > >> > >> Cheers, > >> Andrey > >> > >> _______________________________________________ > >> ghc-devs mailing list > >> ghc-devs(a)haskell.org > >> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs > >> > >> _______________________________________________ > >> ghc-devs mailing list > >> ghc-devs(a)haskell.org > >> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs > > > > > > > > -- > > brandon s allbery kf8nh > > allbery.b(a)gmail.com > > _______________________________________________ > > ghc-devs mailing list > > ghc-devs(a)haskell.org > > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs

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RE: Container type classes
by Andrey Mokhov 30 May '19

30 May '19

Many thanks Iavor, This looks very promising! I played with your encoding a little, but quickly came across type inference issues. The following doesn't compile: add3 :: (Fun s s, Elem s ~ Int) => s -> s add3 = colMap (+1) . colMap (+2) I'm getting: * Could not deduce: Elem a0 ~ Int from the context: (Fun s s, Elem s ~ Int) bound by the type signature for: add3 :: forall s. (Fun s s, Elem s ~ Int) => s -> s Expected type: Elem a0 -> Elem s Actual type: Int -> Int The type variable `a0' is ambiguous Fun s s is supposed to say that the intermediate type is `s` too, but I guess this is not how type class resolution works. Cheers, Andrey -----Original Message----- From: Iavor Diatchki [mailto:iavor.diatchki@gmail.com] Sent: 30 May 2019 22:38 To: Brandon Allbery <allbery.b(a)gmail.com> Cc: Andrey Mokhov <andrey.mokhov(a)newcastle.ac.uk>; Andreas Klebinger <klebinger.andreas(a)gmx.at>; ghc-devs(a)haskell.org Subject: Re: Container type classes This is how you could define `map`. This is just for fun, and to discuss Haskell idioms---I am not suggesting we should do it. Of course, it might be a bit more general than what you'd like---for example it allows defining instances like `Fun IntSet (Set Int)` that, perhaps?, you'd like to disallow: {-# LANGUAGE MultiParamTypeClasses, TypeFamilies #-} import Data.Set (Set) import qualified Data.Set as Set import Data.IntSet (IntSet) import qualified Data.IntSet as ISet class Col t where type Elem t -- ... As in Andreas's example class (Col a, Col b) => Fun a b where colMap :: (Elem a -> Elem b) -> a -> b instance Col (Set a) where type Elem (Set a) = a instance Col IntSet where type Elem IntSet = Int instance Fun IntSet IntSet where colMap = ISet.map instance Ord b => Fun (Set a) (Set b) where colMap = Set.map On Thu, May 30, 2019 at 2:32 PM Brandon Allbery <allbery.b(a)gmail.com> wrote: > > They can, with more work. You want indexed monads, so you can describe types that have e.g. an ordering constraint as well as the Monad constraint. > > On Thu, May 30, 2019 at 5:26 PM Andrey Mokhov <andrey.mokhov(a)newcastle.ac.uk> wrote: >> >> Hi Artem, >> >> >> >> Thanks for the pointer, but this doesn’t seem to be a solution to my challenge: they simply give up on overloading `map` for both Set and IntSet. As a result, we can’t write polymorphic functions over Set and IntSet if they involve any mapping. >> >> >> >> I looked at the prototype by Andreas Klebinger, and it doesn’t include the method `setMap` either. >> >> >> >> Perhaps, Haskell’s type classes just can’t cope with this problem. >> >> >> >> *ducks for cover* >> >> >> >> Cheers, >> >> Andrey >> >> >> >> From: Artem Pelenitsyn [mailto:a.pelenitsyn@gmail.com] >> Sent: 30 May 2019 20:56 >> To: Andrey Mokhov <andrey.mokhov(a)newcastle.ac.uk> >> Cc: ghc-devs(a)haskell.org; Andreas Klebinger <klebinger.andreas(a)gmx.at> >> Subject: Re: Container type classes >> >> >> >> Hi Andrey, >> >> >> >> FWIW, mono-traversable (http://hackage.haskell.org/package/mono-traversable) suggests decoupling IsSet and Funtor-like. >> >> >> >> In a nutshell, they define the IsSet class (in Data.Containers) with typical set operations like member and singleton, union and intersection. And then they tackle a (seemingly) independent problem of mapping monomorphic containers (like IntSet, ByteString, etc.) with a separate class MonoFunctor (in Data.MonoTraversable): >> >> >> >> class MonoFunctor mono where >> omap :: (Element mono -> Element mono) -> mono -> mono >> >> >> >> And gazillion of instances for both polymorphic containers with a fixed type parameter and monomorphic ones. >> >> >> >> -- >> >> Best wishes, >> >> Artem >> >> >> >> On Thu, 30 May 2019 at 20:11, Andrey Mokhov <andrey.mokhov(a)newcastle.ac.uk> wrote: >> >> Hi all, >> >> I tried to use type classes for unifying APIs of several similar data structures and it didn't work well. (In my case I was working with graphs, instead of sets or maps.) >> >> First, you rarely want to be polymorphic over the set representation, because you care about performance. You really want to use that Very.Special.Set.insert because it has the right performance characteristics for your task at hand. I found only *one* use-case for writing polymorphic functions operating on something like IsSet: the testsuite. Of course, it is very nice to write a single property test like >> >> memberInsertProperty x set = (member x (insert x set) == True) >> >> and then use it for testing all set data structures that implement `member` and `insert`. Here you don't care about performance, only about correctness! >> >> However, this approach leads to problems with type inference, confusing error messages, and complexity. I found that it is much nicer to use explicit dictionary passing and write something like this instead: >> >> memberInsertProperty SetAPI{..} x set = (member x (insert x set) == True) >> >> where `member` and `insert` come from the SetAPI record via RecordWildCards. >> >> Finally, I'm not even sure how to create a type class covering Set and IntSet with the following two methods: >> >> singleton :: a -> Set a >> map :: Ord b => (a -> b) -> Set a -> Set b >> >> singleton :: Int -> IntSet >> map :: (Int -> Int) -> IntSet -> IntSet >> >> Could anyone please enlighten me about the right way to abstract over this using type classes? >> >> I tried a few approaches, for example: >> >> class IsSet s where >> type Elem s >> singleton :: Elem s -> s >> map :: Ord (Elem t) => (Elem s -> Elem t) -> s -> t >> >> Looks nice, but I can't define the IntSet instance: >> >> instance IsSet IntSet where >> type Elem IntSet = Int >> singleton = IntSet.singleton >> map = IntSet.map >> >> This fails with: Couldn't match type `t' with `IntSet' -- and indeed, how do I tell the compiler that in the IntSet case s ~ t in the map signature? Shall I add more associated types, or "associated constraints" using ConstraintKinds? I tried and failed, at various stages, repeatedly. >> >> ...And then you discover that there is Set.cartesianProduct :: Set a -> Set b -> Set (a, b), but no equivalent in IntSet and things get even more grim. >> >> Cheers, >> Andrey >> >> _______________________________________________ >> ghc-devs mailing list >> ghc-devs(a)haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs >> >> _______________________________________________ >> ghc-devs mailing list >> ghc-devs(a)haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs > > > > -- > brandon s allbery kf8nh > allbery.b(a)gmail.com > _______________________________________________ > ghc-devs mailing list > ghc-devs(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs

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