L-11 -- 4.2 Associativity Annotations

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4.2 Associativity Annotations

There are two associativity annotations

[] %left assoc
[] %right assoc

which may be used immediately after an operation or predicate declaration. For example, declaring __+__ to be left associative means that t₁ + t₂ + t₃ is parsed as (t₁ + t₂) + t₃, while declaring it to be right associative leads to t₁ + (t₂ + t₃). If there is no associativity annotation for a mixfix symbol, it is not allowed to repeat that symbol without explicit grouping using parentheses.

Due to the CASL summary, section 2.1.2.1, an assoc operation attribute implies a parsing annotation that allows an infix operation to be iterated without explicit grouping parentheses. We choose just %left assoc to be this parsing annotation - %right assoc would also do the job, and the resulting parsed term would be different but semantically equivalent due to associativity of the operation. Thus the choice between %left assoc and %right assoc is arbitrary here, but we have to make a choice to be definite.

CoFI Note: L-11 -- Version: 0.1 -- 11 March 1999.
Comments to roba@informatik.uni-bremen.de