How to use Postgres Functions?

Hi there, I'm trying to call a PostgreSQL function such as the following from within my haskell code, with no success:

CREATE FUNCTION my_fancy_function
       (arg1 integer,
        arg2 character varying) 
RETURNS character varying
LANGUAGE plpgsql
AS $$
BEGIN 
    RETURN "Success"
END
$$;

Initially, I tried using customExpr_ and got this error when attempting to make my own SELECT my_fancy_function(1, 'something'); via a function with (Monoid a, IsString a) as in the docs:

SqlError {sqlState = "42601", sqlExecStatus = FatalError, sqlErrorMsg = "syntax error at or near \"SELECT\"", sqlErrorDetail = "", sqlErrorHint = ""}

Later on I discovered that there is functionCallE, but I could not get it to work. My first attempt was something like this (this does not type check):

functionCall_ ::
  (Projectible be a, BeamSqlBackend be, IsSql99ExpressionSyntax syntax) =>
  Text ->
  [QExpr syntax s a] ->
  QExpr syntax s t
functionCall_ functionName args = QExpr (const $ functionCallE (functionNameE functionName) args)

Further, assuming functionCall_ can be made, would we use it via the function select? I noticed that we don't have SqlExpression as we do for SqlSelect, SqlInsert, and SqlUpdate. Thanks in advance.