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27
Advanced/Lambda Calculus/parts/00 intro.tex
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@ -1,13 +1,13 @@
\section{Definitions} \section{Definitions}
\generic{$\lm$ Notation:} \generic{$\lm$ Notation:}
$\lm$ notation is used to define functions, and looks like $(\lm ~ \text{input} ~ . ~ \text{output})$. \\ $\lm$ notation is used to define functions, and looks like $(\lm ~ \text{input} ~ . ~ \text{output})$. \par
Consider the following statement: Consider the following statement:
$$ $$
I = \lm a . a I = \lm a . a
$$ $$
This tells us that $I$ is a function that takes its input, $a$, to itself. We'll call this the \textit{identity function}. \\ This tells us that $I$ is a function that takes its input, $a$, to itself. We'll call this the \textit{identity function}.
To apply functions, put them next to their inputs. We'll omit the usual parentheses to save space. To apply functions, put them next to their inputs. We'll omit the usual parentheses to save space.
@ -27,7 +27,8 @@ $$
\end{tikzpicture} \end{tikzpicture}
$$ $$
Functions are left-associative: If $A$ and $B$ are functions, $(A~B~\star)$ is equivalent to $((A~B)~\star)$. As usual, we'll use parentheses to group terms if we want to override this order: $(A~(B~\star)) \neq (A~B~\star)$ \\ Functions are left-associative: If $A$ and $B$ are functions, $(A~B~\star)$ is equivalent to $((A~B)~\star)$.
As usual, we'll use parentheses to group terms if we want to override this order: $(A~(B~\star)) \neq (A~B~\star)$ \par
In this handout, all types of parentheses ( $(), [],$ etc ) are equivalent. In this handout, all types of parentheses ( $(), [],$ etc ) are equivalent.
\vfill \vfill
@ -88,8 +89,7 @@ Before we begin, let's review \textit{function composition}. With \say{normal} f
\vspace{1ex} \vspace{1ex}
To demonstrate currying, we'll make a function $C$ in lambda calculus, which takes two functions ($f$ and $g$), an input $x$, and produces $f$ applied to $g$ applied to $x$. \\ To demonstrate currying, we'll make a function $C$ in lambda calculus, which takes two functions ($f$ and $g$), an input $x$, and produces $f$ applied to $g$ applied to $x$. \par
In other words, we want a $C$ so that $C~f~g~x = f~(g~x)$ In other words, we want a $C$ so that $C~f~g~x = f~(g~x)$
\vspace{1ex} \vspace{1ex}
@ -101,7 +101,7 @@ $$
\vspace{1ex} \vspace{1ex}
This looks awfully scary, so let's take this expression apart. \\ This looks awfully scary, so let's take this expression apart. \par
C is a function that takes one argument ($f$) and returns a function (underlined): C is a function that takes one argument ($f$) and returns a function (underlined):
$$ $$
C = \lm f .(~~\tzm{a} \lm g . (\lm x . f(g(x))) \tzm{b}~~) C = \lm f .(~~\tzm{a} \lm g . (\lm x . f(g(x))) \tzm{b}~~)
@ -138,7 +138,7 @@ This last function $\lm x. f(g(x))$ takes one argument, and returns $f(g(x))$. S
\vspace{2ex} \vspace{2ex}
So, currying allows us to create multivariable functions by nesting single-variable functions. \\ So, currying allows us to create multivariable functions by nesting single-variable functions. \par
As you saw above, such expressions can get very long. We'll use a bit of shorthand to make them more palatable. If we have an expression with repeated function definitions, we'll combine their arguments under one $\lm$. As you saw above, such expressions can get very long. We'll use a bit of shorthand to make them more palatable. If we have an expression with repeated function definitions, we'll combine their arguments under one $\lm$.
\vspace{1ex} \vspace{1ex}
@ -154,11 +154,12 @@ $$
\vspace{1ex} \vspace{1ex}
This is only notation. \textbf{Curried functions are not multivariable functions!} They must be evaluated one variable at a time, just like their un-curried version. Substituting all curried variables in one go may cause errors, as you'll see below. This is only notation. \textbf{Curried functions are not multivariable functions!} They must be evaluated one variable at a time, just like their un-curried version.
Substituting all curried variables in one go may cause errors, as you'll see below.
\problem{} \problem{}
Evaluate $C~M~I~\star$ \\ Evaluate $C~M~I~\star$ \par
Then, evaluate $C~I~M~I$ \\ Then, evaluate $C~I~M~I$ \par
\hint{Place parentheses first. Remember, function application is left-associative.} \hint{Place parentheses first. Remember, function application is left-associative.}
@ -174,8 +175,8 @@ $I = \lm a.a = \lm b.b = \lm \heartsuit . \heartsuit = ...$
Variables inside functions are \say{scoped.} We must take care to keep separate variables separate. Variables inside functions are \say{scoped.} We must take care to keep separate variables separate.
For example, take the functions \\ For example, take the functions \par
$A = \lm a b . a$ \\ $A = \lm a b . a$ \par
$B = \lm b . b$ $B = \lm b . b$
\vspace{2ex} \vspace{2ex}
@ -214,7 +215,7 @@ $$
Now, we correctly find that $(A~B~I) = (\lm bc . c)~I = \lm c . c = B = I$. Now, we correctly find that $(A~B~I) = (\lm bc . c)~I = \lm c . c = B = I$.
\problem{} \problem{}
Let $C = \lm abc.b$ \\ Let $C = \lm abc.b$ \par
Reduce $(C~a~c~b)$. Reduce $(C~a~c~b)$.
\begin{solution} \begin{solution}

16
Advanced/Lambda Calculus/parts/01 combinators.tex
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@ -1,17 +1,17 @@
\section{Combinators} \section{Combinators}
\definition{} \definition{}
A \textit{free variable} in a $\lm$-expression is a variable that isn't bound to any input. \\ A \textit{free variable} in a $\lm$-expression is a variable that isn't bound to any input. \par
For example, $b$ is a free variable in $\lm a. b$. The same is true of $\star$ in any of the previous pages. For example, $b$ is a free variable in $\lm a. b$. The same is true of $\star$ in any of the previous pages.
A \textit{combinator} is a function with no free variables. A \textit{combinator} is a function with no free variables.
\definition{The Kestrel} \definition{The Kestrel}
Notable combinators are often named after birds.\hspace{-0.5ex}\footnotemark{} We've already met a few: \\ Notable combinators are often named after birds.\hspace{-0.5ex}\footnotemark{} We've already met a few: \par
The \textit{Idiot}, $I = \lm a.a$ \\ The \textit{Idiot}, $I = \lm a.a$ \par
The \textit{Mockingbird}, $M = \lm f.ff$ \\ The \textit{Mockingbird}, $M = \lm f.ff$ \par
The \textit{Cardinal}, $C = \lm fgx.(~ f(g(x)) ~)$ \\ The \textit{Cardinal}, $C = \lm fgx.(~ f(g(x)) ~)$
\footnotetext{Raymond Smullyan's \textit{To Mock a Mockingbird} is responsible for this.} \footnotetext{Raymond Smullyan's \textit{To Mock a Mockingbird} is responsible for this.}
@ -22,17 +22,17 @@ $$
K = \lm ab . a K = \lm ab . a
$$ $$
\problem{} \problem{}
What does the Kestrel do? Explain in plain English. \\ What does the Kestrel do? Explain in plain English. \par
\hint{What is $(K~\heartsuit~\star)$?} \hint{What is $(K~\heartsuit~\star)$?}
\vspace{2cm} \vspace{2cm}
\problem{} \problem{}
Reduce $(K~I)$ to derive the \textit{Kite}. How does the Kite compare to the Kestrel? \\ Reduce $(K~I)$ to derive the \textit{Kite}. How does the Kite compare to the Kestrel? \par
We'll call the Kite KI. We'll call the Kite KI.
\begin{solution} \begin{solution}
$\text{KI} = \lm ab . b$. \\ $\text{KI} = \lm ab . b$.
\end{solution} \end{solution}
\vfill \vfill

10
Advanced/Lambda Calculus/parts/02 boolean.tex
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@ -4,16 +4,16 @@ The Kestrel selects its first argument, and the Kite selects its second. This \s
\vspace{1ex} \vspace{1ex}
Let $T = K\phantom{I} = \lm ab . a$ \\ Let $T = K\phantom{I} = \lm ab . a$ \par
Let $F = KI = \lm ab . b$ \\ Let $F = KI = \lm ab . b$
\problem{} \problem{}
Write a function $\text{NOT}$ so that $(\text{NOT} ~ T) = F$ and $(\text{NOT}~F) = T$. \\ Write a function $\text{NOT}$ so that $(\text{NOT} ~ T) = F$ and $(\text{NOT}~F) = T$. \par
\hint{What is $(T~\heartsuit~\star)$? How about $(F~\heartsuit~\star)$?} \hint{What is $(T~\heartsuit~\star)$? How about $(F~\heartsuit~\star)$?}
\begin{solution} \begin{solution}
$\text{NOT} = \lm a . (a~F~T)$ \\ $\text{NOT} = \lm a . (a~F~T)$
\end{solution} \end{solution}
\vfill \vfill
@ -51,7 +51,7 @@ Write functions $\text{AND}$, $\text{OR}$, and $\text{XOR}$ that satisfy the fol
\vfill \vfill
\problem{} \problem{}
To complete our boolean algebra, write the boolean equality check EQ. \\ To complete our boolean algebra, write the boolean equality check EQ. \par
What inputs should it take? What outputs should it produce? What inputs should it take? What outputs should it produce?
\begin{solution} \begin{solution}

58
Advanced/Lambda Calculus/parts/03 numbers.tex
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@ -1,10 +1,10 @@
\section{Numbers} \section{Numbers}
Since the only objects we have in $\lm$-calculus are functions, it's natural to think of quantities as \textit{adverbs} (once, twice, thrice,...) rather than \textit{nouns} (one, two, three ...) \\ Since the only objects we have in $\lm$-calculus are functions, it's natural to think of quantities as \textit{adverbs} (once, twice, thrice,...) rather than \textit{nouns} (one, two, three ...)
\vspace{1ex} \vspace{1ex}
We'll start with zero. If our numbers are \textit{once,} \textit{twice,} and \textit{twice}, it may make sense to make zero \textit{don't}. \\ We'll start with zero. If our numbers are \textit{once,} \textit{twice,} and \textit{twice}, it may make sense to make zero \textit{don't}. \par
Here's our \textit{don't} function: given a function and an input, don't apply the function to the input. Here's our \textit{don't} function: given a function and an input, don't apply the function to the input.
$$ $$
0 = \lm fa.a 0 = \lm fa.a
@ -18,13 +18,13 @@ Write $1$, $2$, and $3$. We will call these \textit{Church numerals}.\hspace{-0.
\footnotetext{after Alonzo Church, the inventor of lambda calculus and these numerals. He was Alan Turing's thesis advisor.} \footnotetext{after Alonzo Church, the inventor of lambda calculus and these numerals. He was Alan Turing's thesis advisor.}
\begin{solution} \begin{solution}
$1$ calls a function once on its argument: \\ $1$ calls a function once on its argument: \par
$1 = \lm fa . (f~a)$. $1 = \lm fa . (f~a)$.
\vspace{1ex} \vspace{1ex}
Naturally, \\ Naturally, \par
$2 = \lm fa . [~f~(f~a)~]$ \\ $2 = \lm fa . [~f~(f~a)~]$ \par
$3 = \lm fa . [~f~(f~(f~a))~]$ $3 = \lm fa . [~f~(f~(f~a))~]$
\vspace{1ex} \vspace{1ex}
@ -49,7 +49,7 @@ What is $(4~I)~\star$?
\vfill \vfill
\problem{} \problem{}
What is $(3~NOT~T)$? \\ What is $(3~NOT~T)$? \par
How about $(8~NOT~F)$? How about $(8~NOT~F)$?
\vfill \vfill
@ -57,13 +57,13 @@ How about $(8~NOT~F)$?
\pagebreak \pagebreak
\problem{} \problem{}
This handout may remind you of Professor Oleg's handout on Peano's axioms. Good. \\ This handout may remind you of Professor Oleg's handout on Peano's axioms. Good. \par
Recall the tools we used to build the natural numbers: \\ Recall the tools we used to build the natural numbers: \par
We had a zero element and a \say{successor} operation so that $1 \coloneqq S(0)$, $2 \coloneqq S(1)$, and so on. We had a zero element and a \say{successor} operation so that $1 \coloneqq S(0)$, $2 \coloneqq S(1)$, and so on.
\vspace{1ex} \vspace{1ex}
Create a successor operation for the Church numerals. \\ Create a successor operation for the Church numerals. \par
\hint{A good signature for this function is $\lm nfa$, or more clearly $\lm n.\lm fa$. Do you see why?} \hint{A good signature for this function is $\lm nfa$, or more clearly $\lm n.\lm fa$. Do you see why?}
\begin{solution} \begin{solution}
@ -83,14 +83,14 @@ Verify that $S(0) = 1$ and $S(1) = 2$.
\pagebreak \pagebreak
Assume that only Church numerals will be passed to the functions in the following problems. \\ Assume that only Church numerals will be passed to the functions in the following problems. \par
We make no promises about their output if they're given anything else. We make no promises about their output if they're given anything else.
\problem{} \problem{}
Define a function ADD that adds two Church numerals. Define a function ADD that adds two Church numerals.
\begin{solution} \begin{solution}
$\text{ADD} = \lm mn . (m~S~n) = \lm mn . (n~S~m)$ \\ $\text{ADD} = \lm mn . (m~S~n) = \lm mn . (n~S~m)$
\end{solution} \end{solution}
\begin{instructornote} \begin{instructornote}
@ -111,8 +111,8 @@ Define a function ADD that adds two Church numerals.
\vfill \vfill
\problem{} \problem{}
Adding is nice, but we can do better. \\ Adding is nice, but we can do better. \par
Design a function MULT that multiplies two numbers. \\ Design a function MULT that multiplies two numbers. \par
\hint{The easy solution uses ADD, the elegant one doesn't. Find both!} \hint{The easy solution uses ADD, the elegant one doesn't. Find both!}
\begin{solution} \begin{solution}
@ -126,22 +126,22 @@ Design a function MULT that multiplies two numbers. \\
\problem{} \problem{}
Define the functions $Z$ and $NZ$. $Z$ should reduce to $T$ if its input was zero, and $F$ if it wasn't. \\ Define the functions $Z$ and $NZ$. $Z$ should reduce to $T$ if its input was zero, and $F$ if it wasn't. \par
$NZ$ does the opposite. $Z$ and $NZ$ should look fairly similar. $NZ$ does the opposite. $Z$ and $NZ$ should look fairly similar.
\vspace{1ex} \vspace{1ex}
\begin{solution} \begin{solution}
$Z\phantom{N} = \lm n .[n~(\lm a.F)~T]$\\ $Z\phantom{N} = \lm n .[n~(\lm a.F)~T]$ \par
$NZ = \lm n .[n~(\lm a.T)~F]$ $NZ = \lm n .[n~(\lm a.T)~F]$
\end{solution} \end{solution}
\vfill \vfill
\problem{} \problem{}
Data structures will be useful. Design an expression PAIR that constructs two-value tuples. \\ Data structures will be useful. Design an expression PAIR that constructs two-value tuples. \par
For example, say $A = \text{PAIR}~1~2$. Then, \\ For example, say $A = \text{PAIR}~1~2$. Then, \par
$(A~T)$ should reduce to $1$ \\ $(A~T)$ should reduce to $1$ \par
$(A~F)$ should reduce to $2$ $(A~F)$ should reduce to $2$
\begin{solution} \begin{solution}
@ -149,21 +149,21 @@ $(A~F)$ should reduce to $2$
\end{solution} \end{solution}
\vfill \vfill
From now on, I'll write (PAIR $A$ $B$) as $\langle A,B \rangle$. \\ From now on, I'll write (PAIR $A$ $B$) as $\langle A,B \rangle$. \par
Like currying, this is only notation. The underlying $\lm$ logic remains the same. Like currying, this is only notation. The underlying $\lm$ logic remains the same.
\pagebreak \pagebreak
\problem{}<shiftadd> \problem{}<shiftadd>
Write a function $H$, which we'll call \say{shift and add.} \\ Write a function $H$, which we'll call \say{shift and add.} \par
It does exactly what it says on the tin: \\ It does exactly what it says on the tin:
\vspace{1ex} \vspace{1ex}
Given an input pair, it should shift its right argument left, then add one. \\ Given an input pair, it should shift its right argument left, then add one. \par
$H~\langle 0, 1 \rangle$ should reduce to $\langle 1, 2\rangle$ \\ $H~\langle 0, 1 \rangle$ should reduce to $\langle 1, 2\rangle$ \par
$H~\langle 1, 2 \rangle$ should reduce to $\langle 2, 3\rangle$ \\ $H~\langle 1, 2 \rangle$ should reduce to $\langle 2, 3\rangle$ \par
$H~\langle 10, 4 \rangle$ should reduce to $\langle 4, 5\rangle$ \\ $H~\langle 10, 4 \rangle$ should reduce to $\langle 4, 5\rangle$
\begin{solution} \begin{solution}
$H = \lm p . \Bigl\langle~(p~F)~,~S(p~F)~\Bigr\rangle$ $H = \lm p . \Bigl\langle~(p~F)~,~S(p~F)~\Bigr\rangle$
@ -177,8 +177,8 @@ $H~\langle 10, 4 \rangle$ should reduce to $\langle 4, 5\rangle$ \\
\vfill \vfill
\problem{} \problem{}
Design a function $D$ that un-does $S$. That means \\ Design a function $D$ that un-does $S$. That means \par
$D(1) = 0$, $D(2) = 1$, etc. $D(0)$ should be zero. \\ $D(1) = 0$, $D(2) = 1$, etc. $D(0)$ should be zero. \par
\hint{$H$ will help you make an elegant solution.} \hint{$H$ will help you make an elegant solution.}
\begin{solution} \begin{solution}
@ -186,8 +186,8 @@ $D(1) = 0$, $D(2) = 1$, etc. $D(0)$ should be zero. \\
\end{solution} \end{solution}
\begin{solution} \begin{solution}
Here's a different solution. \\ Here's a different solution. \par
Can you figure out how it works? \\ Can you figure out how it works?
\vspace{1ex} \vspace{1ex}

14
Advanced/Lambda Calculus/parts/04 recursion.tex
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@ -12,7 +12,7 @@ We cannot re-create this in lambda notation. Functions in lambda calculus are \t
\vspace{1ex} \vspace{1ex}
As an example, consider the statement $A = \lm a. A~a$ \\ As an example, consider the statement $A = \lm a. A~a$ \par
This means \say{write $(\lm a.A~a)$ whenever you see $A$.} However, $A$ is \textit{inside} what we're rewriting. We'd fall into infinite recursion before even starting our $\beta$-reduction! This means \say{write $(\lm a.A~a)$ whenever you see $A$.} However, $A$ is \textit{inside} what we're rewriting. We'd fall into infinite recursion before even starting our $\beta$-reduction!
\begin{instructornote} \begin{instructornote}
@ -20,7 +20,7 @@ This means \say{write $(\lm a.A~a)$ whenever you see $A$.} However, $A$ is \text
\vspace{4ex} \vspace{4ex}
Say we have a device that reduces a $\lm$ expression to $\beta$-normal form. We give it an expression, and the machine simplifies it as much as it can and spits out the result. \\ Say we have a device that reduces a $\lm$ expression to $\beta$-normal form. We give it an expression, and the machine simplifies it as much as it can and spits out the result.
\vspace{1ex} \vspace{1ex}
@ -28,12 +28,12 @@ This means \say{write $(\lm a.A~a)$ whenever you see $A$.} However, $A$ is \text
\end{instructornote} \end{instructornote}
\problem{} \problem{}
Write an expression that resolves to itself. \\ Write an expression that resolves to itself. \par
\note{Your answer should be short and sweet.} \note{Your answer should be short and sweet.}
\vspace{1ex} \vspace{1ex}
This expression is often called $\Omega$, after the last letter of the Greek alphabet. \\ This expression is often called $\Omega$, after the last letter of the Greek alphabet. \par
$\Omega$ useless on its own, but gives us a starting point for recursion. $\Omega$ useless on its own, but gives us a starting point for recursion.
\begin{solution} \begin{solution}
@ -41,20 +41,20 @@ $\Omega$ useless on its own, but gives us a starting point for recursion.
\vspace{1ex} \vspace{1ex}
An uninspired mathematician might call the Mockingbird $\omega$, \say{little omega}. \\ An uninspired mathematician might call the Mockingbird $\omega$, \say{little omega}.
\end{solution} \end{solution}
\vfill \vfill
\definition{} \definition{}
This is the \textit{Y-combinator}, easily the most famous $\lm$ expression. \\ This is the \textit{Y-combinator}, easily the most famous $\lm$ expression. \par
You may notice that it's just $\Omega$, put to work. You may notice that it's just $\Omega$, put to work.
$$ $$
Y = \lm f . (\lm x . f(x~x))(\lm x . f(x~x)) Y = \lm f . (\lm x . f(x~x))(\lm x . f(x~x))
$$ $$
\problem{} \problem{}
What does this thing do? \\ What does this thing do? \par
Evaluate $Y f$. Evaluate $Y f$.
\vfill \vfill

19
Advanced/Lambda Calculus/parts/05 challenges.tex
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@ -1,18 +1,19 @@
\section{Challenges} \section{Challenges}
Do \ref{Yfac} first, then finish the rest in any order. \\ Do \ref{Yfac} first, then finish the rest in any order.
\problem{}<Yfac> \problem{}<Yfac>
Design a recursive factorial function using $Y$. \\ Design a recursive factorial function using $Y$.
\vfill \vfill
\problem{} \problem{}
Design a non-recursive factorial function. \\ Design a non-recursive factorial function. \par
\note{This one is a lot easier than \ref{Yfac}, but I don't think it will help you solve it.} \note{This one is a lot easier than \ref{Yfac}, but I don't think it will help you solve it.}
\problem{} \problem{}
Using pairs, make a \say{list} data structure. Define a GET function, so that $\text{GET}~L~n$ reduces to the nth item in the list. $\text{GET}~L~0$ should give the first item in the list, and $\text{GET}~L~1$, the \textit{second}. \\ Using pairs, make a \say{list} data structure. Define a GET function, so that $\text{GET}~L~n$ reduces to the nth item in the list.
$\text{GET}~L~0$ should give the first item in the list, and $\text{GET}~L~1$, the \textit{second}. \par
Lists have a defined length, so you should be able to tell when you're on the last element. Lists have a defined length, so you should be able to tell when you're on the last element.
\problem{} \problem{}
@ -61,15 +62,15 @@ Write a MOD $a$ $b$ function that reduces to the remainder of $a \div b$.
\vspace{1ex} \vspace{1ex}
\say{is last} is a boolean, true iff this is the last item in the list. \\ \say{is last} is a boolean, true iff this is the last item in the list. \par
\say{item} is the thing you're storing \\ \say{item} is the thing you're storing \par
\say{next...} is another one of these list fragments. \\ \say{next...} is another one of these list fragments.
It doesn't matter what \say{next} is in the last list fragment. A dedicated \say{is last} slot allows us to store ANY function in this list. It doesn't matter what \say{next} is in the last list fragment. A dedicated \say{is last} slot allows us to store ANY function in this list.
\vspace{1ex} \vspace{1ex}
Here, $\text{GET} = \lm nL.[(n~L~F)~T~F$] \\ Here, $\text{GET} = \lm nL.[(n~L~F)~T~F$]
This will break if $n$ is out of range. This will break if $n$ is out of range.
\end{solution} \end{solution}
@ -77,5 +78,5 @@ Write a MOD $a$ $b$ function that reduces to the remainder of $a \div b$.
\vfill \vfill
\problem{Bonus} \problem{Bonus}
Play with \textit{Lamb}, an automatic lambda expression evaluator. \\ Play with \textit{Lamb}, an automatic lambda expression evaluator. \par
\url{https://git.betalupi.com/Mark/lamb} \url{https://git.betalupi.com/Mark/lamb}