Uniting Is Only Allowed in strong Positions
In ALGOL 68 uniting is the coercion that produces a union from a constituent mode, for example:
mode ibool = union (int, bool); co an ibool is an int or a bool co ibool a = true; co the bool value true is united to an ibool coIn standard ALGOL 68 uniting was possible in firm or strong contexts, so for example could be applied to the operands of formulas:
op istrue = (ibool a) bool: ...; if istrue 1 co legal because 1 (int) can be united to ibool co then ...The ALGOL 68-R implementers found this gave too many ambiguous situations so restricted the uniting coercion to strong contexts.
The effects of this restriction were rarely important and, if necessary, could be worked around by using a cast to provide a strong context at the required point in the program.
Read more about this topic: ALGOL 68R, Restrictions in The Language Compiled
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