Documentation

Preferences recognized by the application.

How to determine the count of (expected) results

How to pass parameters to stored procedures.

How to return the mutated data.

From https://tools.ietf.org/html/rfc7240#section-4.2

How to handle duplicate values.

Whether to commit or roll back transactions.

Parse HTTP headers based on RFC7240[1] to identify preferences.

One header with comma-separated values can be used to set multiple preferences:

>>> pPrint $ fromHeaders [("Prefer", "resolution=ignore-duplicates, count=exact")]
Preferences
    { preferResolution = Just IgnoreDuplicates
    , preferRepresentation = Nothing
    , preferParameters = Nothing
    , preferCount = Just ExactCount
    , preferTransaction = Nothing
    }

Multiple headers can also be used:

>>> pPrint $ fromHeaders [("Prefer", "resolution=ignore-duplicates"), ("Prefer", "count=exact")]
Preferences
    { preferResolution = Just IgnoreDuplicates
    , preferRepresentation = Nothing
    , preferParameters = Nothing
    , preferCount = Just ExactCount
    , preferTransaction = Nothing
    }

If a preference is set more than once, only the first is used:

>>> preferTransaction $ fromHeaders [("Prefer", "tx=commit, tx=rollback")]
Just Commit

This is also the case across multiple headers:

>>> :{
  preferResolution . fromHeaders $
    [ ("Prefer", "resolution=ignore-duplicates")
    , ("Prefer", "resolution=merge-duplicates")
    ]
:}
Just IgnoreDuplicates

Preferences not recognized by the application are ignored:

>>> preferResolution $ fromHeaders [("Prefer", "resolution=foo")]
Nothing

Preferences can be separated by arbitrary amounts of space, lower-case header is also recognized:

>>> pPrint $ fromHeaders [("prefer", "count=exact,    tx=commit   ,return=minimal")]
Preferences
    { preferResolution = Nothing
    , preferRepresentation = Just None
    , preferParameters = Nothing
    , preferCount = Just ExactCount
    , preferTransaction = Just Commit
    }

Header to indicate that a preference has been applied.

>>> toAppliedHeader MergeDuplicates
("Preference-Applied","resolution=merge-duplicates")