Lambda Diagrams
Lambda Diagrams are a graphical notation for closed lambda terms,
in which abstractions (lambdas) are represented by horizontal lines,
variables by vertical lines emanating down from their binding lambda,
and applications by horizontal links connecting the leftmost variables.
In the alternative style, applications link the nearest deepest variables,
for a more stylistic, if less uniform, look.
The following table shows diagrams of identity, the booleans, some standard combinators,
some Church numerals, the predecessor function on Church numerals, and Omega.
term  definition  diagram  alternative 
I  λx.x 
 
K/true  λx.λy.x 
 
false  λx.λy.y 
 
S  λx.λy.λz.(x z)(y z) 


Y  λf.(λx.x x)(λx.f(x x)) 


2  λf.λx.f(f x) 


3  λf.λx.f(f(f x)) 


4  λf.λx.f(f(f(f x))) 


pred  λn.λf.λx.n(λg.λh.h(g f))(λu.x)(λu.u) 


fac  λn λf.n(λf.λn.n(f(λf.λx.n f(f x))))(λx.f)(λx.x) 


Ω  (λx.x x)(λx.x x) 


Curiously, the alternative Omega diagram somewhat resembles an (upsidedown) Omega.
And here, on a larger scale, is a prime number sieve (alternative style):
which reduces to an infinite list of booleans that starts out as
Dimensions and complexity
In terms of pixels, a diagram's width is one less than 4 times the number of variables
(vertical lines),
and its height is one more than twice the maximum number of nested abstractions and applications
(one less for alternaitve diagrams with multiple variables).
The size of the binary encoding of a term is closely related to
the graphical complexity: it is exactly twice the number of lines
plus the number of (4way) intersections.
Reduction
Betareduction on lambda diagrams can be shown in several steps, as demonstrated in this reduction
from Y=λf.(λx.x x)(λx.f(x x)) to λf.(λx.f(x x))(λx.f(x x))
initial term  
show application of abstraction  
show bound variables and argument  
expand function body  
to make room for substitution  
substitute argument for variables  
final term  
In the third frame, we show only the part of bound variables below abstractions, e.g. when
applying , the function body x(λy.x) shows as .
Diagrams In Motion
Paul Brauner has produced some awesome videos of beta reductions.
Related work
Dave Keenan has a comprehensive online paper
To Dissect a Mockingbird:
A Graphical Notation for the Lambda Calculus with Animated Reduction,
which partly inspired this page.
In his Master thesis, Viktor Massalõgin discusses 4 existing graphical notations before introducing his own "bubble" notation. Figure 3 on page 10 shows 4 depictions of the fixpoint combinator (which differs from Y above in one beta reduction), while the bubble form is in Figure 5 on page 13.
Note
The diagram in the title, produced by the postscript code below, is a slightly deformed alternative Y diagram made to look like a Y.
%!PSAdobe2.0 EPSF2.0
%%BoundingBox:0 0 118 110
/m{moveto}def/l{lineto}def/c{concat 6 m 0 6 l 7 8 m 0 8 l l l 3 6 l 2 6 m 7 6 l
3 4 m 6 4 l 6 6 l stroke}def 3 0 0 0 1[8 4 0 8 60 9]c 3 2 0 2 2[1 1 0 1 0 0]c
Back to my Binary Lambda Calculus page.