How to implent CONVERT ( data_type [ ( length ) ] , expression ) function in postgreSQL

Dear

In SQLServer, there'are two functions to converte an expression of one data type to another.
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 RegardsRohtodeveloper