PostgreSQL syntax error in parameterized query on "date $1" - postgresql

Question

Trying to parameterize my SQL queries (using libpq function PQexecParams), I was stuck on a syntax error:
SELECT date $1
The error is:
ERROR: syntax error at or near "$1"

Answer 1

Prepared statements
The explanation for this can be found in the chapter Constants of Other Types of the manual:
The ::, CAST(), and function-call syntaxes can also be used to specify
run-time type conversions of arbitrary expressions, as discussed in
Section 4.2.9. To avoid syntactic ambiguity, the type 'string' syntax
can only be used to specify the type of a simple literal constant.
Another restriction on the type 'string' syntax is that it does not
work for array types; use :: or CAST() to specify the type of an array
constant.
Bold emphasis mine.
Parameters for prepared statements are not actually sting literals but typed values, so you cannot use the form type 'string'. Use one of the other two forms to cast the value to a different type, like you found yourself already.
Example:
PREPARE foo AS SELECT $1::date;
EXECUTE foo('2005-1-1');
Similar for PQexecParams in the libpq C library
The documentation:
... In the SQL command text, attach an explicit cast to the parameter
symbol to show what data type you will send. For example:
SELECT * FROM mytable WHERE x = $1::bigint;
This forces parameter $1 to be treated as bigint, whereas by default
it would be assigned the same type as x. Forcing the parameter type
decision, either this way or by specifying a numeric type OID, is
strongly recommended. ...
The alternative, as mentioned in the quote above, is to pass the OIDs of respective data types with paramTypes[] - if you actually need the cast. In most cases it should work just fine to let Postgres derive data types from the query context.
paramTypes[]
Specifies, by OID, the data types to be assigned to the parameter
symbols. If paramTypes is NULL, or any particular element in the array
is zero, the server infers a data type for the parameter symbol in the
same way it would do for an untyped literal string.
You can get the OID of data types from the system catalog pg_type:
SELECT oid FROM pg_type WHERE typname = 'date';
You must use the correct internal type name. For instance: int4 for integer.
Or with a convenience cast to regtype:
SELECT 'date'::regtype::oid;
This is more flexible as known aliases for the type name are accepted as well. For instance: int4, int or integer for integer.

Answer 2

The solution is to use a type cast instead of date:
SELECT $1::date