A value of type
'a Lazy.t is a deferred computation, called a suspension, that has a result of type
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.
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.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
module Monad_infix : sig ... end
val return : 'a -> 'a t
return v returns the (trivial) computation that returns 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
Pervasives.ignore. Some monads still do
let ignore = ignore_m for historical reasons.
module Let_syntax : sig ... end
These are convenient to have in scope when programming with a monad:
val force : 'a t -> 'a
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.
val force_val : 'a t -> 'a
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
val from_fun : (unit -> 'a) -> 'a t
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
val from_val : 'a -> 'a t
from_val v returns an already-forced suspension of
v can be any expression). Essentially,
from_val expr is the same as
let var = expr in lazy
val is_val : 'a t -> bool
is_val x returns
x has already been forced and did not raise an exception.
module T_unforcing : sig ... end
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.