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)