I'm trying to load data from a SQLite database, but I can't know the data format, since it's user provided.

This works if all columns are of the same type:

(entries :: [[Text]]) <- query_ connection "select * from users"

But what can I do if the columns have different types (e.g. INTEGER or REAL)?

CodePudding user response：

TLDR: Use entries :: [[SQLData]]

You seem to be using sqlite-simple. In that package, the type of query_ is

query_ :: FromRow r => Connection -> Query -> IO [r]

query_ uses the FromRow and (indirectly) FromField typeclasses. These are helpers that transform incoming sqlite row and column data to the format expected by the user.

However, these transformations might fail when the source and output types are not convertible.

The SQLData represents the "raw" untransformed sqlite datatypes. Converting a column value to SQLData is a no-op and never fails. And using [SQLData] for the rows never fails either, because lists can handle any number of columns (unlike, say, a (SQLData,SQLData) tuple).

Therefore, when dealing with unknown schemas, we can use [SQLData] as the row type and then pattern-match on the SQLData values to discover what are the actual sqlite types.