Automatically adding (or removing) a prefix to a record labels in Purescript


Thanks to the awesome heterogeneous library’s dark magic powers, it’s now very easy to do a lot of operations on non heterogeneous collections.

It’s common in Haskell to have prefixed record fields labels, to avoid name clashing. For example:

data User = User { userName :: Text, userPassword :: Password }

In Purescript this is not necessary, and I was starting to be annoyed by the fact that I had to use unnecessary prefixes in the data structures I was using to send data to the backend.

To be honest, I have no idea yet if heterogeneous is necessary for this, but it helped me get to a solution pretty easily.

Using FoldingWithIndex, I wrote a function that goes through a record’s fields and builds a new record prefixing a specified string to all the labels.

I’ll try to explain what’s happening, even though some things are still not 100% clear to me.

The first step is to create a data structure. Heterogeneous needs this to dispatch constraints since the record’s fields are not all of the same type.

sym is the prefix we’re going to add to the labels, and SProxy is a data type used to pass symbol information around.

data PrefixProps sym = PrefixProps (SProxy sym)

Then we write an instance for FoldingWithIndex. The actual function is pretty simple: it folds through the record’s fields and uses Record.builder to create a new record using the prefixed labels. The real magic is happening in the signature:

  • IsSymbol newsym means that newsym is a symbol
  • Append presym sym newsym means that our new symbol is the result of prepending the prefix to the old symbol
  • Row.Lacks newsym rb means that the new prefixed symbol is not already present as a record label
  • Row.Cons newsym a rb rc means that rc is the result of adding a field with label newsym and value a to rb

Then there’s the class FoldingWithIndex representing a fold, with the arguments being our data type, the current symbol, the accumulator Builder, the current value and the result Builder.

instance prefixProps ::
  ( IsSymbol newsym
  , Append presym sym newsym
  , Row.Lacks newsym rb
  , Row.Cons newsym a rb rc
  ) =>
  FoldingWithIndex
    (PrefixProps presym)
    (SProxy sym)
    (Builder { | ra } { | rb })
    a
    (Builder { | ra } { | rc })
  where
  foldingWithIndex (PrefixProps _) _ rin a =
(_ >>> Builder.insert (SProxy :: SProxy newsym) a) rin

The last thing is the actual function that we are going to use. It calls hfoldlWithIndex using the class instance we just wrote, and then uses Record.builder to create the actual record.

add ::
  forall rin rout pre.
  HFoldlWithIndex (PrefixProps pre) (Builder {} {}) { | rin } (Builder {} { | rout }) =>
  SProxy pre ->
  { | rin } ->
  { | rout }
add pre = flip Builder.build {} <<< hfoldlWithIndex (PrefixProps pre) identity

Done! remove is basically the same, and here’s how they can be used:

foo :: { bar :: Int, baz :: Boolean }
foo = { bar: 1, baz: true }


bar :: { prebar :: Int, prebaz :: Boolean }
bar = add (SProxy :: SProxy "pre") foo

baz :: { bar :: Int, baz :: Boolean }
baz = remove (SProxy :: SProxy "pre") bar

You can find the code here.

If you want to learn more about heterogenous, watch this video

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