Documentation

A collection type that can be converted from a list of strings.

Instances should use the convert method of the Result class to perform conversion of each element of the collection.

This example instance demonstrates how to convert a two-column row into a Haskell pair. Each field in the metadata is paired up with each value from the row, and the two are passed to convert.

instance (Result a, Result b) => QueryResults (a,b) where
    convertResults [fa,fb] [va,vb] = (a,b)
        where !a = convert fa va
              !b = convert fb vb
    convertResults fs vs  = convertError fs vs 2

Notice that this instance evaluates each element to WHNF before constructing the pair. By doing this, we guarantee two important properties:

• Keep resource usage under control by preventing the construction of potentially long-lived thunks.
• Ensure that any ResultError that might arise is thrown immediately, rather than some place later in application code that cannot handle it.

You can also declare Haskell types of your own to be instances of QueryResults.

data User = User { firstName :: String, lastName :: String }

instance QueryResults User where
    convertResults [fa,fb] [va,vb] = User $ a * b
        where !a = convert fa va
              !b = convert fb vb
    convertResults fs vs  = convertError fs vs 2
 

Throw a ConversionFailed exception, indicating a mismatch between the number of columns in the Field and row, and the number in the collection to be converted to.