Version 6 (modified by YitzGale, 6 years ago) (diff)

Typo: binging => binding

Records in Haskell

This Yesod blog post, and accompanying Reddit discussion brought to the surface again the thorny issue of records in Haskell.

There are two rather different sets of issues:

  • The narrow issue: namespacing for record field names. Currently in Haskell two records in the same module can't share a field name. This is sometimes extremely painful. This page is about the narrow issue.
  • The broad issue: first class record types. In Haskell there is no "record type" per se. Rather, you can simply give names to the fields of a constructor. Records are not extensible and here is no polymorphism on records.

This page focues exclusively on the first, narrow issue of dismbiguating record field names. We have a separate Wiki page, ExtensibleRecords, on the broad issue of first class record types.

On this page I'd like to summarise the problem, and specify alternative designs. So far it is mostly a skeleton: please fill it out. The idea is to hold a discussion by email (ghc-users?) but to collect results (alternative designs, trade-offs, pros and cons) here, because email threads quickly get lost. Simon PJ.

The problem: record name spacing

(Quoting the Yesod blog.) Consider

data Record = Record { a :: String }
data RecordClash = RecordClash { a :: String }

Compiling this file results in:

    Multiple declarations of `Main.a'
    Declared at: record.hs:1:24

In the Persistent data store library, Yesod works around the issue by having the standard of prefixing every record field with the record name (recordA and recordClashA). But besides being extremely verbose, it also limits us from experimenting with more advanced features like a partial record projection or an unsaved and saved record type.

The verbose name-spacing required is an in-your-face, glaring weakness telling you there is something wrong with Haskell. This issue has been solved in almost every modern programming languages, and there are plenty of possible solutions available to Haskell.


I know of two sorts of solutions:

  1. Better name spacing
  2. Type directed name resolution

Are there any other approaches?

Better name spacing

In Haskell, you can look at an occurrence of any identifier f or M.f and decide where it is bound without thinking about types at all. Broadly speaking it works like this:

  • For qualified names, M.f, find an import that binds M.f.
  • For unqualified names, f, find the innermost binding of f; or, if that takes you to top level, look for top level binding of f or an import that binds f.

If there is ambiguity (eg two imports both import something called f) then an error is reported. And that's what happens for the Record and RecordClash example above.

So one solution for record field names is to specify more precisely which one you mean. There are two schools of thought:

  • Optionally use the type name. So you could say Record.a or RecordClash.a rather than a, to specify which field selector you mean. Apart from verbosity the difficulty here is that it's hard to know whether you are writing <module-name>.f or <type-name>.f. That is, is Record the name of a type or of a module? (Currently it legally could be both.)
  • Use the module name space mechanism; after all that's what it's for. But putting each record definition in its own module is a bit heavyweight. So maybe we need local modules (just for name space control) and local import declarations. Details are unclear.

Anyone who likes these designs, please fill out a detailed design, either here or on another page.

Problems with using the module namespace mechanism

Suppose I have 112 hand-crafted data types in my project (e.g. see attachment 51369.txt), this creates a lot of conflicts in field names and constructor names. For example:

data Comment = Comment {
      commentId           :: CommentId
    , commentContent      :: Content
    , commentReviewId     :: ReviewId
    , commentSubmissionId :: SubmissionId
    , commentConferenceId :: ConferenceId
    , commentDate         :: ISODate
    , commentReviewerNumber :: Int
  } deriving (Show)

This is a real type in my project. It has fields like “id”, “content”, “reviewId”, “submissionId”, “date”. There are seven other data types that have a field name “submissionId”. There are 15 with “conferenceId”. There are 7 with “content”. And so on. This is just to demonstrate that field clashes do occur a lot in a nontrivial project.

It also demonstrates that if you propose to put each of these 112 types into a separate module, you are having a laugh. I tried this around the 20 type mark and it was, apart from being very slow at compiling, very tedious to work with. Creating and editing these modules was a distracting and pointless chore.

It also demonstrated, to me, that qualified imports are horrible when used on a large scale. It happened all the time, that'd I'd import, say, 10 different data types all qualified. Typing map ( . BarMu.thisField) and foo Bar.Zot{x=1,y=2} becomes tedious and distracting, especially having to add every type module when I want to use a type. And when records use other types in other modules, you have a lot of redundancy. With the prefixing paradigm I'd write fooId and barMuThisField, which is about as tedious but there is at least less . confusion and no need to make a load of modules and import lines. Perhaps local modules would solve half of this problem. Still have to write “ bar” rather than “mu bar”, but it'd be an improvement.

I also have 21 Enum types which often conflict. I end up having to include the name of the type in the constructor, or rewording it awkwardly. I guess I should put these all in separate modules and import qualified, too. Tedious, though. At least in this case languages like C# and Java also require that you type EnumName.EnumValue, so c‘est la vie.

Type directed name resolution

All of the name-space mechanisms require some level of user-supplied disambiguation: if there are two fields a in scope, you must use a qualified name to disambiguate them. What is tantalising about this is that the type of the argument immediately specifies which one you mean. There is really no ambiguity at all, so it is frustrating to have to type qualified names to redundantly specify that information. Object-oriented languages take for granted this form of type-directed disambiguation.

One particular way of integrating this idea into Haskell is called Type Directed Name Resolution (TDNR). Proposed a couple of years ago, the Haskell community didn't like it much. (But I still do; SLPJ.)

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