set_fixity : string -> fixity -> unit
Allows the fixity of tokens to be updated.
The set_fixity function is used to change the fixity of single tokens. It implements this functionality rather crudely. When called on to set the fixity of t to f, it removes all rules mentioning t from the global (term) grammar, and then adds a new rule to the grammar. The new rule maps occurrences of t with the given fixity to terms of the same name.
This function fails if the new fixity causes a clash with existing rules, as happens if the precedence level of the specified fixity is already taken by rules using a fixity of a different type. Even if the application of set_fixity succeeds, it may cause the next subsequent application of the Term parsing function to complain about precedence conflicts in the operator precedence matrix. These problems may cause the parser to behave oddly on terms involving the token whose fixity was set. Excessive parentheses will usually cure even these problems.
After a new constant is defined, set_fixity can be used to give it an appropriate parse status:
   - val thm = Psyntax.new_recursive_definition
                  prim_recTheory.num_Axiom "f"
                  (Term`(f 0 n = n) /\ (f (SUC n) m = SUC (SUC (f n m)))`);
   > val thm =
       |- (!n. f 0 n = n) /\ !n m. f (SUC n) m = SUC (SUC (f n m))
       : thm
   - set_fixity "f" (Infixl 500);
   > val it = () : unit
   - thm;
   > val it =
       |- (!n. 0 f n = n) /\ !n m. SUC n f m = SUC (SUC (n f m)) : thm
The same function can be used to alter the fixities of existing constants:
   - val t = Term`2 + 3 + 4 - 6`;
   > val t = `2 + 3 + 4 - 6` : term
   - set_fixity "+" (Infixr 501);
   > val it = () : unit
   - t;
   > val it = `(2 + 3) + 4 - 6` : term
   - dest_comb (Term`3 - 1 + 2`);
   > val it = (`$- 3`, `1 + 2`) : term * term

This function is of no use if multiple-token rules (such as those for conditional expressions) are desired, or if the token does not correspond to the name of the constant or variable that is to be produced. (For the latter case, use set_mapped_fixity.)

As with other functions in the Parse structure, there is a companion temp_set_fixity function, which has the same effect on the global grammar, but which does not cause this effect to persist when the current theory is exported.

HOL  Kananaskis-14