Inessential Guide to data-accessor

by Edward Z. Yang

data-accessor is a package that makes records not suck. Instead of this code:

newRecord = record {field = newVal}

You can write this:

newRecord = field ^= newVal
          $ record

In particular, (field ^= newVal) is now a value, not a bit of extra syntax, that you can treat as a first-class citizen.

I came across this module while attempting to use Chart (of criterion fame) to graph some data. I didn't recognize it at first, though; it was only after playing around with code samples did I realize that ^= was not a combinator that Chart had invented for its own use (as opposed to the potpourri of -->, <+>, ||| and friends you might see in an xmonad.hs). When utilized with Template Haskell, Data.Accessor represents something of a replacement for the normal record system, and so it's useful to know when a module speaks this other language. Signs that you're in a module using Data.Accessor:

  • Use of the ^= operator in code samples
  • All of the records have underscores suffixed, such as plot_lines_title_
  • Template Haskell gobbledygook (including type variables that look like x[acGI], especially in the "real" accessors that Template Haskell generated).
  • Unqualified T data types floating around. (As Brent Yorgey tells me, this is a Henning-ism in which he will define a type T or typeclass C intended to be used only with a qualified import, but Haddock throws away this information. You can use :t in GHC to get back this information if you're not sure.)

Once you've identified that a module is indeed using Data.Accessor, you've won most of the battle. Here is a whirlwind tutorial on how to use records that use data-accessor.

Interpreting types. An accessor (represented by the type Data.Accessor.T r a) is defined to be a getter (r -> a) and setter (a -> r -> r). r is the type of the record, and a is the type of the value that can be retrieved or set. If Template Haskell was used to generate the definitions, polymorphic types inside of a and r will frequently be universally quantified with type variables that x[acGI], don't worry too much about them; you can pretend they're normal type variables. For the curious, these are generated by the quotation monad in Template Haskell).

Accessing record fields. The old way:

fieldValue = fieldName record

You can do things several ways with Data.Accessor:

fieldValue = getVal fieldname record
fieldValue = record ^. fieldname

Setting record fields. The old way:

newRecord = record {fieldName = newValue}

The new ways:

newRecord = setVal fieldName newValue record
newRecord = fieldName ^= newValue $ record

Accessing and setting sub-record fields. The old ways:

innerValue = innerField (outerField record)
newRecord = record {
  outerField = (outerField record) {
    innerField = newValue

The new ways (this is bit reminiscent of semantic editor combinators):

innerValue = getVal (outerField .> innerField) record
newRecord = setVal (outerField .> innerField) newValue record

There are also functions for modifying records inside the state monad, but I'll leave those explanations for the Haddock documentation. Now go forth and, erm, access your data in style!