Converting an expression of one data type to another

Dear

I'm doing a job about converting an expression of one data type to another.

In SQLServer, there'are two functions to do this job.

1. CAST ( expression AS data_type [ ( length ) ] )

2. CONVERT ( data_type [ ( length ) ] , expression )

However, In PostgreSQL, there's only the CAST ( expression AS data_type [ ( length ) ] ) function. I have tried the following two ways to implenting the CONVERT ( data_type [ ( length ) ] , expression ) function, but both are failed.

1. CREATE FUNCTION ..... 

The function's arguments can only be expressions but not data_type . 

2. Modifying the gram.y .....

The CONVERT ( data_type [ ( length ) ] , expression ) is in grammer conflict with the PostgreSQL self's convert(data,src_encoding_name,dest_encoding_name) function. And the PostgreSQL self's convert(data,src_encoding_name,dest_encoding_name) function cannot be used.

I wonder whether there's a better way to solve this problem. 

Any help will be appreciated.

Best Regards

Rohtodeveloper