1. Carlos Varela Rennselaer Polytechnic Institute September 4, 2015
Lambda Calculus (PDCS 2) alpha-renaming, beta reduction, eta conversion, applicative and normal evaluation orders, Church-Rosser theorem, combinators, higher-order programming, recursion combinator, numbers, booleans
Carlos Varela
Rennselaer Polytechnic Institute
September 4, 2015
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2. Lambda Calculus Syntax and Semantics
The syntax of a -calculus expression is as follows:
e ::= v variable
| v.e functional abstraction
| (e e) function application
The semantics of a -calculus expression is called beta-reduction:
(x.E M)  E{M/x}
where we alpha-rename the lambda abstraction E if necessary to avoid capturing free variables in M.
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3. Function Composition in Lambda Calculus
S: x.(s x) (Square)
I: x.(i x) (Increment)
C: f.g.x.(f (g x)) (Function Composition)
((C S) I)
((f.g.x.(f (g x)) x.(s x)) x.(i x))
 (g.x.(x.(s x) (g x)) x.(i x))
 x.(x.(s x) (x.(i x) x))
 x.(x.(s x) (i x))
 x.(s (i x))
Recall semantics rule:
(x.E M)  E{M/x}
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4. Order of Evaluation in the Lambda Calculus
Does the order of evaluation change the final result?
Consider:
x.(x.(s x) (x.(i x) x))
There are two possible evaluation orders:
 x.(x.(s x) (i x))
 x.(s (i x))
and:
 x.(s (x.(i x) x))
Is the final result always the same?
Recall semantics rule:
(x.E M)  E{M/x}
Applicative Order
Normal Order
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5. Church-Rosser Theorem
If a lambda calculus expression can be evaluated in two different ways and both ways terminate, both ways will yield the same result. e
e e2 e’
Also called the diamond or confluence property.
Furthermore, if there is a way for an expression evaluation to terminate, using normal order will cause termination.
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6. Order of Evaluation and Termination
Consider:
(x.y (x.(x x) x.(x x)))
There are two possible evaluation orders:
 (x.y (x.(x x) x.(x x)))
and:
 y
In this example, normal order terminates whereas applicative order does not.
Recall semantics rule:
(x.E M)  E{M/x}
Applicative Order
Normal Order
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7. Free and Bound Variables
The lambda functional abstraction is the only syntactic construct that binds variables. That is, in an expression of the form:
v.e
we say that occurrences of variable v in expression e are bound. All other variable occurrences are said to be free.
E.g.,
(x.y.(x y) (y w))
Bound Variables
Free Variables
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8. This reduction erroneously captures the free occurrence of y.
Why -renaming?
Alpha renaming is used to prevent capturing free occurrences of variables when reducing a lambda calculus expression, e.g.,
(x.y.(x y) (y w))
y.((y w) y)
This reduction erroneously captures the free occurrence of y.
A correct reduction first renames y to z, (or any other fresh variable) e.g.,
 (x.z.(x z) (y w))
 z.((y w) z)
where y remains free.
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9. -renaming
Alpha renaming is used to prevent capturing free occurrences of variables when beta-reducing a lambda calculus expression.
In the following, we rename x to z, (or any other fresh variable):
(x.(y x) x)
(z.(y z) x)
Only bound variables can be renamed. No free variables can be captured (become bound) in the process. For example, we cannot alpha-rename x to y. α→
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10. b-reduction (x.E M) E{M/x}
Beta-reduction may require alpha renaming to prevent capturing free variable occurrences. For example:
(x.y.(x y) (y w))
(x.z.(x z) (y w))
z.((y w) z)
Where the free y remains free. α→ b→
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11. h-conversion x.(E x) E if x is not free in E. For example:
(x.y.(x y) (y w))
(x.z.(x z) (y w))
z.((y w) z)
(y w) α→ b→ h→
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12. Combinators
A lambda calculus expression with no free variables is called a combinator. For example:
I: x.x (Identity)
App: f.x.(f x) (Application)
C: f.g.x.(f (g x)) (Composition)
L: (x.(x x) x.(x x)) (Loop)
Cur: f.x.y.((f x) y) (Currying)
Seq: x.y.(z.y x) (Sequencing--normal order)
ASeq: x.y.(y x) (Sequencing--applicative order)
where y denotes a thunk, i.e., a lambda abstraction wrapping the second expression to evaluate.
The meaning of a combinator is always the same independently of its context.
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13. Combinators in Functional Programming Languages
Functional programming languages have a syntactic form for lambda abstractions. For example the identity combinator:
x.x
can be written in Oz as follows:
fun {$ X} X end
in Haskell as follows: \x -> x
and in Scheme as follows: (lambda(x) x)
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14. Currying Combinator in Oz
The currying combinator can be written in Oz as follows:
fun {$ F}
fun {$ X}
fun {$ Y}
{F X Y}
end
It takes a function of two arguments, F, and returns its curried version, e.g.,
{{{Curry Plus} 2} 3}  5
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15. Recursion Combinator (Y or rec)
Suppose we want to express a factorial function in the l calculus.
1 n=0
f(n) = n! =
n*(n-1)! n>0
We may try to write it as:
f: n.(if (= n 0) 1
(* n (f (- n 1))))
But f is a free variable that should represent our factorial function.
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16. Recursion Combinator (Y or rec)
We may try to pass f as an argument (g) as follows:
f: g.n.(if (= n 0) 1
(* n (g (- n 1))))
The type of f is:
f: (Z  Z)  (Z  Z)
So, what argument g can we pass to f to get the factorial function?
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17. Recursion Combinator (Y or rec)
f: (Z  Z)  (Z  Z)
(f f) is not well-typed.
(f I) corresponds to:
1 n=0
f(n) =
n*(n-1) n>0
We need to solve the fixpoint equation:
(f X) = X
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18. Recursion Combinator (Y or rec)
(f X) = X
The X that solves this equation is the following:
X: (lx.(g.n.(if (= n 0) 1
(* n (g (- n 1))))
ly.((x x) y))
lx.(g.n.(if (= n 0)
ly.((x x) y)))
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19. Recursion Combinator (Y or rec)
X can be defined as (Y f), where Y is the recursion combinator.
Y: lf.(lx.(f ly.((x x) y))
lx.(f ly.((x x) y)))
Y: lf.(lx.(f (x x))
lx.(f (x x)))
You get from the normal order to the applicative order recursion combinator by h-expansion (h-conversion from right to left).
Applicative Order
Normal Order
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20. Natural Numbers in Lambda Calculus
|0|: x.x (Zero)
|1|: x.x.x (One) …
|n+1|: x.|n| (N+1)
s: ln.lx.n (Successor)
(s 0)
(n.x.n x.x)
 x.x.x
Recall semantics rule:
(x.E M)  E{M/x}
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21. Booleans and Branching (if) in l Calculus
|true|: x.y.x (True)
|false|: x.y.y (False)
|if|: b.lt.le.((b t) e) (If)
(((if true) a) b)
(((b.t.e.((b t) e) x.y.x) a) b)
 ((t.e.((x.y.x t) e) a) b)
 (e.((x.ly.x a) e) b)
 ((x.ly.x a) b)
(y.a b) a
Recall semantics rule:
(x.E M)  E{M/x}
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22. Exercises PDCS Exercise 2.11.7 (page 31).
Prove that your addition operation is correct using induction.
PDCS Exercise (page 31). Test your representation of booleans in Haskell.
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