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A value of type 'a Lazy.t is a deferred computation, called a suspension, that has a result of type 'a.
The special expression syntax lazy (expr) makes a suspension of the computation of expr, without computing expr itself yet. "Forcing" the suspension will then compute expr and return its result.
Note: lazy_t is the built-in type constructor used by the compiler for the lazy keyword. You should not use it directly. Always use Lazy.t instead.
Note: Lazy.force is not thread-safe. If you use this module in a multi-threaded program, you will need to add some locks.
Note: if the program is compiled with the -rectypes option, ill-founded recursive definitions of the form let rec x = lazy x or let rec x = lazy(lazy(...(lazy x))) are accepted by the type-checker and lead, when forced, to ill-formed values that trigger infinite loops in the garbage collector and other parts of the run-time system. Without the -rectypes option, such ill-founded recursive definitions are rejected by the type-checker.
t >>= f returns a computation that sequences the computations represented by two monad elements. The resulting computation first does t to yield a value v, and then runs the computation returned by f v.
ignore_m t is map t ~f:(fun _ -> ()). ignore_m used to be called ignore, but we decided that was a bad name, because it shadowed the widely used Caml.ignore. Some monads still do let ignore = ignore_m for historical reasons.
force x forces the suspension x and returns its result. If x has already been forced, Lazy.force x returns the same value again without recomputing it. If it raised an exception, the same exception is raised again. Raise Undefined if the forcing of x tries to force x itself recursively.
Like force except that force_val x does not use an exception handler, so it may be more efficient. However, if the computation of x raises an exception, it is unspecified whether force_val x raises the same exception or Undefined.
from_fun f is the same as lazy (f ()) but slightly more efficient if f is a variable. from_fun should only be used if the function f is already defined. In particular it is always less efficient to write from_fun (fun () -> expr) than lazy
expr.
from_val v returns an already-forced suspension of v (where v can be any expression). Essentially, from_val expr is the same as let var = expr in lazy
var.
This type offers a serialization function sexp_of_t that won't force its argument. Instead, it will serialize the 'a if it is available, or just use a custom string indicating it is not forced. Note that this is not a round-trippable type, thus the type does not expose of_sexp. To be used in debug code, while tracking a Heisenbug, etc.