| ↔ Order by name | ↔ Order by year | ↔ Order by year reverse | ↔ Order by Type | ↔ Order by Sammet category |
| OPAL | ⊕ 1993 | ↓ ↑ ⊕ Specialised Languages | ⊕ Goguen algebra | ⊕ Germany | Algebraic functional language |
| OPAL-0 | ⊕ 1993 | ↓ ↑ ⊕ String and List Processing | ⊕ Goguen algebra | ⊕ Germany | Algebraic functional language |
| OPAL-1 | ⊕ 1993 | ↓ ↑ ⊕ Specialised Languages | ⊕ Goguen algebra | ⊕ Germany | Algebraic functional language |