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:
```plpgsql
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](https://hackage.haskell.org/package/beam-core-0.10.3.1/docs/Database-Beam-Query-CustomSQL.html):
```
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):
```haskell
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.