The foreign-type tells the compiler how to interpret a T data type in order to pass it to the foreign call. The general categories of Pascal data types are numeric, string, record, enumerated, set of.
| Pascal Type | T3 Type | Foreign Type Spec | |
| Numeric | |||
| integer8 | fixnum | rep/integer-8-s | |
| binteger | fixnum | rep/integer-8-u | |
| integer16 | fixnum | rep/integer-16-s | |
| pinteger | fixnum | rep/integer-16-u | |
| integer | fixnum | rep/integer | |
| linteger | fixnum | rep/integer | |
| real | unimplemented | ||
| double | flonum | rep/extend | |
| String | |||
| string | string | rep/string | |
| string | text | rep/extend | |
| univ_pointer | string | rep/string-pointer | |
| univ_pointer | text | rep/extend-pointer | |
| Record | |||
| record | extend | rep/extend | |
| Miscellaneous | |||
| char | char | rep/char | |
| boolean | boolean | rep/integer-8-s |
Beware that if a T string is being used as an out parameter the offset field of the string must be 0 (the string must never have been chdr!'ed).
Record structures are represented by byte-vectors of the appropriate size.