Prolog realizes high-order programming with meta-calling. The core predicate of this is call/1, which simply calls its argument. This can be used to define higher-order predicates such as ignore/1 or forall/2. The call/N construct calls a closure with N-1 additional arguments. This is used to define higher-order predicates such as the maplist/N family or foldl/N.
The problem with higher order predicates based on call/N is that the
additional arguments are always added to the end of the closure's
argument list. This often requires defining trivial helper predicates to
get the argument order right. For example, if you want to add a common
postfix to a list of atoms you need to apply
atom_concat(In,Postfix,Out), but maplist(x(PostFix),ListIn,ListOut)
calls x(PostFix,In,Out). This is where this library comes
in, which allows us to write
?- maplist([In,Out]>>atom_concat(In,'_p',Out), [a,b], ListOut). ListOut = [a_p, b_p].
The {...} specifies which variables are shared
between the lambda and the context. This allows us to write the code
below. Without the
{PostFix} a free variable would be passed to atom_concat/3.
add_postfix(PostFix, ListIn, ListOut) :-
maplist({PostFix}/[In,Out]>>atom_concat(In,PostFix,Out),
ListIn, ListOut).
This introduces the second application area of lambda expressions:
the ability to stop binding variables in the context. This features
shines when combined with bagof/3
or setof/3 where
you normally have to specify the the variables in whose binding you are not
interested using the
Var^Goal construct (marking Var as existential
quantified). Lambdas allow doing the reverse: specify the variables in
which you are interested.
Lambda expressions use the syntax below
{...}/[...]>>Goal.
The {...} optional part is used for lambda-free
variables. The order of variables doesn't matter hence the {...}
set notation.
The [...] optional part lists lambda parameters. Here
order of variables matters hence the list notation.
As / and >> are standard infix
operators, no new operators are added by this library. An advantage of
this syntax is that we can simply unify a lambda expression with
Free/Parameters>>Lambda to access each of its
components. Spaces in the lambda expression are not a problem although
the goal may need to be written between ()'s. Goals that are qualified
by a module prefix also need to be wrapped inside parentheses.
Combined with library(apply_macros), library(yall)
allows writing one-liners for many list operations that have the same
performance as hand written code.
The module name, yall, stands for Yet Another Lambda Library.
This module implements Logtalk's lambda expressions syntax. The development of this module was sponsored by Kyndi, Inc.
call(Lambda,A1,...),
but arguments are reordered according to the list Parameters:
length(Parameters) arguments from A1,
... are unified with (a copy of) Parameters, which may
share them with variables in Lambda.| Parameters | is either a plain list of
parameters or a term
{Free}/List. Free represents variables that are
shared between the context and the Lambda term. This is
needed for compiling Lambda expressions. |
Free/[]>>Lambda. This is the same as
applying call/N on Lambda, except that only variables
appearing in Free are bound by the call. For example
p(1,a).
p(2,b).
?- {X}/p(X,Y).
X = 1;
X = 2.
This can in particularly be combined with bagof/3 and setof/3 to select particular variables to be concerned rather than using existential quantification (^/2) to exclude variables. For example, the two calls below are equivalent.
setof(X, Y^p(X,Y), Xs)
setof(X, {X}/p(X,_), Xs)