Pack maybe -- prolog/maybe.pl

is_just(+Maybe:maybe) is semidet

True if Maybe is just(_). Useful for include/3.

is_nothing(+Maybe:maybe) is semidet

True if Maybe is nothing. Useful for exclude/3.

just_value(+Just:maybe(T), -Value:T) is det

just_value(-Just:maybe(T), +Value:T) is det

True if Just wraps Value. Throws an exception if Just is nothing.

maybe_value(+Maybe:maybe(T), -Value:T) is det

True if Maybe is just(Value); fails for nothing.

call_maybe(:Goal, ?Value:T, -Maybe:maybe(T)) is multi

If call(Goal) succeeds, Maybe=just(Value), otherwise Maybe=nothing. Goal typically binds Value. If Goal produces multiple solutions on backtracking, so will call_maybe/3. In that case, Maybe will never be nothing.

For example,

?- L = [a,b,c], call_maybe(L=[H|_], H, MaybeHead).
H = a,
MaybeHead = just(a).

?- L = [], call_maybe(L=[H|_], H, MaybeHead).
MaybeHead = nothing.

Of course, that particular example could have been implemented with maybe_list/2 instead.

maybe_list(?Maybe:maybe(T), ?List:list(T)) is det

Relates a List to a Maybe value. An empty list is equivalent to nothing. A non-empty list is equivalent to just(Head).

maybe_default_value(+Maybe:maybe(T), +Default:T, -Value:T) is det

maybe_default_value(-Maybe:maybe(T), +Default:T, +Value:T) is multi

True if Maybe wraps Value with a Default for the nothing case.

default_maybe_value(+Default:T, +Maybe:maybe(T), -Value:T) is det

default_maybe_value(+Default:T, -Maybe:maybe(T), +Value:T) is multi

Like maybe_default_value/3 with different argument order. This argument order is convenient for maplist/3 like so:

maplist(default_maybe_value(7), Maybes, DefaultedValues).

map_maybe(+Goal, ?Maybe0:maybe, ?Maybe:maybe)

True if call(Goal, Value0, Value) succeeds for just values. Goal is not called for nothing values, which remain unchanged. "Use the source" for a clearer explanation.

fold_maybe(+Goal, ?Maybe:maybe, ?Accum0, ?Accum)

nothing leaves Accum0=Accum while just relates them via Goal. "Use the source" for a clearer explanation.