This section lists, in ascii order, all the Schelog primitives, with a brief explanation for each. Each entry is identified as either procedure, macro, predicate, goal, or flag. (Note that predicates and goals are also procedures. We nevertheless use the more specific names because of their importance to Schelog programming.)
Following Prolog style, a predicate’s arity is also noted in
its title. Thus, %</2 means that %< takes two
arguments. Variable-arity predicates use an asterisk
instead of a number, eg, %and/*.
%/=/2 | [predicate] |
%/= is the negation of %=.
The goal (%/= E1 E2) succeeds if E1 can not be unified
with E2.
%/==/2 | [predicate] |
%/== is the negation of %==.
The goal (%/== E1 E2) succeeds if E1 and E2 are not
identical.
%</2 | [predicate] |
The goal (%< E1 E2) succeeds if E1 and E2 are bound to
numbers and E1 is less than E2.
%<=/2 | [predicate] |
The goal (%<= E1 E2) succeeds if E1 and E2 are bound to
numbers and E1 is less than or equal to E2.
%=/2 | [predicate] |
The goal (%= E1 E2) succeeds if E1 can be unified with
E2. Any resulting bindings for logic variables are kept.
%=/=/2 | [predicate] |
The goal (%=/= E1 E2) succeeds if E1 and E2 are bound to
numbers and E1 is not equal to E2.
%=:=/2 | [predicate] |
The goal (%=:= E1 E2) succeeds if E1 and E2 are bound to
numbers and E1 is equal to E2.
%==/2 | [predicate] |
The goal (%== E1 E2) succeeds if E1 is identical
to E2. They should be structurally equal. If containing
logic variables, they should have the same variables in the
same position. Unlike a %=-call, this goal will not bind
any logic variables.
%>/2 | [predicate] |
The goal (%> E1 E2) succeeds if E1 and E2 are bound to
numbers and E1 is greater than E2.
%>=/2 | [predicate] |
The goal (%>= E1 E2) succeeds if E1 and E2 are bound to
numbers and E1 is greater than or equal to E2.
%and/* | [macro] |
The goal (%and G ...) succeeds if all the goals
G, ..., succeed.
%append/3 | [predicate] |
The goal (%append E1 E2 E3) succeeds if E3 is unifiable
with the list obtained by appending E1 and E2
%assert | [macro] |
The form (%assert Pname (V ...) C ...) adds the clauses
C, ..., to the end of the predicate that is the value of
the Scheme variable Pname. The variables V, ..., are
local logic variables for C, ...
%assert‑a | [macro] |
Like %assert, but adds the new clauses to the front
of the existing predicate.
%bag‑of/3 | [predicate] |
The goal (%bag‑of E1 G E2) unifies with E2 the bag
(multiset)
of all the
instantiations of E1 for which goal G succeeds.
%bag‑of‑1/3 | [predicate] |
Similar to %bag‑of, but fails if the bag is empty.
%compound/1 | [predicate] |
The goal (%compound E) succeeds if E is a non-atomic
structure, ie, a vector or a list.
%constant/1 | [predicate] |
The goal (%compound E) succeeds if E is an atomic
structure, ie, not a vector or a list.
%copy/2 | [predicate] |
The goal (%copy F S) unifies with S a copy of the
frozen structure in F.
%empty‑rel/* | [predicate] |
The goal (%empty‑rel E ...) always fails. The value
%empty‑rel is used as a starting value for predicates
that can later be enhanced with %assert and %assert‑a.
%fail | [goal] |
The goal %fail always fails.
%free‑vars | [macro] |
The form (%free‑vars (V ...) G) identifies
the occurrences of the variables V, ..., in goal
G as free. It is used to avoid existential quantification
in calls to set predicates (%bag‑of, %set‑of, &c.).
%freeze/2 | [predicate] |
The goal (%freeze S F) unifies with F a new frozen
version of the structure in S. Freezing implies that all
the unbound variables are preserved. F can henceforth be
used as bound object with no fear of its variables
getting bound by unification.
%if‑then‑else/3 | [predicate] |
The goal (%if‑then‑else G1 G2 G3) tries G1 first: if it
succeeds, tries G2; if not, tries G3.
%is/2 | [predicate] |
The goal (%is E1 E2) unifies with E1 the result of
evaluating E2 as a Scheme expression. E2 may contain
logic variables, which are dereferenced automatically.
Fails if E2 contains unbound logic variables.
(Note: Unlike other Schelog predicates, %is is implemented
as a macro and not a procedure.)
%let | [macro] |
The form (%let (V ...) E ...) introduces V, ..., as
lexically scoped logic variables to be used in E, ...
%melt/2 | [predicate] |
The goal (%melt F S) unifies S with the thawed
(original) form of the frozen structure in F.
%melt‑new/2 | [predicate] |
The goal (%melt‑new F S) unifies S with a thawed
copy of the frozen structure in F. This means
new logic variables are used for unbound logic variables in
F
%member/2 | [predicate] |
The goal (%member E1 E2) succeeds if E1 is a member
of the list in E2.
%nonvar/1 | [predicate] |
%nonvar is the negation of %var.
The goal (%nonvar E) succeeds if E is completely
instantiated, ie, it has no unbound variable in it.
%not/1 | [predicate] |
The goal (%not G) succeeds if G fails.
%more | [procedure] |
The thunk %more produces more instantiations of the
variables in the most recent %which-form that satisfy the
goals in that %which-form. If no more solutions can
be found, %more returns #f.
%or/* | [macro] |
The goal (%or G ...) succeeds if one of G, ..., tried
in that order, succeeds.
%rel | [macro] |
The form (%rel (V ...) C ...) creates a predicate object.
Each clause C is of the form [(E ...) G ...], signifying
that the goal created by applying the predicate object to
anything that matches (E ...) is deemed to succeed if all
the goals G, ..., can, in their turn, be shown to succeed.
%repeat/0 | [predicate] |
The goal (%repeat) always succeeds (even on retries).
Used for failure-driven loops.
*schelog‑use‑occurs‑check?* | [flag] |
If the global flag
*schelog‑use‑occurs‑check?* is false (the default),
Schelog’s unification will not use the occurs check.
If it is true, the occurs check is enabled.
%set‑of/3 | [predicate] |
The goal (%set‑of E1 G E2) unifies with E2 the set
of all the
instantiations of E1 for which goal G succeeds.
%set‑of‑1/3 | [predicate] |
Similar to %set‑of, but fails if the set is empty.
%true | [goal] |
The goal %true succeeds. Fails on retry.
%var/1 | [predicate] |
The goal (%var E) succeeds if E is not completely
instantiated, ie, it has at least one unbound variable in
it.
%which | [macro] |
The form (%which (V ...) G ...) returns an instantiation
of the variables V, ..., that satisfies all of G,
... If G, ..., cannot be satisfied, returns #f.
Calling the thunk %more produces more
instantiations, if available.
_ (underscore) | [procedure] |
A thunk that produces a new logic variable. Can be
used in situations where we want a logic variable but
don’t want to name it. (%let, in contrast, introduces new
lexical names for the logic variables it creates.)