enable_monad : string -> unit
Enables a particular monadic type for use with do/od syntax.
A call to enable_monad mname, where mname is an SML value of type string, enables the stored information about monad mname to govern the interpretation of the do/od syntax. If multiple monads are enabled, normal overloading resolution will decide between them.
Fails if mname is not the name of a stored monad in the internal database (which can be examined with a call to monadsyntax.all_monads(). Will have little effect if monad syntax has not been generally enabled with a prior call to enable_monadsyntax.
In what follows, oHD is the function which maps a non-empty list to SOME applied to that list’s first element, and the empty list to NONE. The ++ is the monad choice function (the option monad has a notion of failure). Thus, the function below that is bound to SML variable f is one that either increments the first element of a list and returns that value, or returns 0.
> enable_monadsyntax(); enable_monad "option";
val it = () : unit
val it = () : unit

> val f = “λl. do x <- oHD l; return (x + 1); od ++ return 0”
val f = “λl. do x <- oHD l; SOME (x + 1); od ++ SOME 0” : term

> EVAL “^f [3; 10]”;
val it = ⊢ (λl. do x <- oHD l; SOME (x + 1) od ⧺ SOME 0) [3; 10] = SOME 4:

> EVAL “^f []”;
val it = ⊢ (λl. do x <- oHD l; SOME (x + 1) od ⧺ SOME 0) [] = SOME 0: thm
Note how the return keyword is not printed as such by the parser; it would be too confusing if all occurrences of common functions such as SOME were printed as return.
As with other parsing and printing functions, there is a temp_enable_monad function whose changes to the parser and printer do not persist to descendent theories.
HOL  Kananaskis-14