Format SqlRational as a decimal ...

[rickowens]

... in order to avoid this error when trying to insert a rational
value into the database:

SqlError {seState = "22P02", seNativeError = 7, seErrorMsg = "execute: PGRES_FATAL_ERROR: ERROR: invalid input syntax for type numeric: "603 % 10"\n"}

Apparently, the Show instance isn't good enough. This implementation
technically losses some precision in some cases, but then Haskell's
Rational is larger than PostgreSQL's numeric, so that is fundamentally
unavoidable.

[rickowens]

cc @MHova @jonkraut

[rickowens]

So, to talk a little bit more about this, what is happening here is that I have a value 60.3, represented as a Haskell Rational type. I convert that value into a SqlValue using the SqlRational data constructor, which in turn gets used to fill in a positional argument of a prepared statement. HDBC-postgresql seems to use the information to render a real sql statement, and it appears to use the Convertible SqlValue String instance to implement this rendering, which itself delegates to instance Show Rational, which does not produce a string that can be interpreted as valid sql.

cc @hesselink