%% $Id$

\chapter{Constructive Type Theory}

\index{Constructive Type Theory|(}

Martin-L\"of's Constructive Type Theory \cite{martinlof84,nordstrom90} can

be viewed at many different levels.  It is a formal system that embodies

the principles of intuitionistic mathematics; it embodies the

interpretation of propositions as types; it is a vehicle for deriving

programs from proofs.  

Thompson's book~\cite{thompson91} gives a readable and thorough account of

Type Theory.  Nuprl is an elaborate implementation~\cite{constable86}.

{\sc alf} is a more recent tool that allows proof terms to be edited

directly~\cite{alf}.

Isabelle's original formulation of Type Theory was a kind of sequent

calculus, following Martin-L\"of~\cite{martinlof84}.  It included rules for

building the context, namely variable bindings with their types.  A typical

judgement was

\[   a(x@1,\ldots,x@n)\in A(x@1,\ldots,x@n) \; 

    [ x@1\in A@1, x@2\in A@2(x@1), \ldots, x@n\in A@n(x@1,\ldots,x@{n-1}) ]

\]

This sequent calculus was not satisfactory because assumptions like

`suppose $A$ is a type' or `suppose $B(x)$ is a type for all $x$ in $A$'

could not be formalized.  

The theory~\thydx{CTT} implements Constructive Type Theory, using

natural deduction.  The judgement above is expressed using $\Forall$ and

$\Imp$:

\[ \begin{array}{r@{}l}

     \Forall x@1\ldots x@n. &

          \List{x@1\in A@1; 

                x@2\in A@2(x@1); \cdots \; 

                x@n\in A@n(x@1,\ldots,x@{n-1})} \Imp \\

     &  \qquad\qquad a(x@1,\ldots,x@n)\in A(x@1,\ldots,x@n) 

    \end{array}

\]

Assumptions can use all the judgement forms, for instance to express that

$B$ is a family of types over~$A$:

\[ \Forall x . x\in A \Imp B(x)\;{\rm type} \]

To justify the {\CTT} formulation it is probably best to appeal directly

to the semantic explanations of the rules~\cite{martinlof84}, rather than

to the rules themselves.  The order of assumptions no longer matters,

unlike in standard Type Theory.  Contexts, which are typical of many modern

type theories, are difficult to represent in Isabelle.  In particular, it

is difficult to enforce that all the variables in a context are distinct.

\index{assumptions!in {\CTT}}

The theory does not use polymorphism.  Terms in {\CTT} have type~\tydx{i}, the

type of individuals.  Types in {\CTT} have type~\tydx{t}.

\begin{figure} \tabcolsep=1em  %wider spacing in tables

\begin{center}

\begin{tabular}{rrr} 

  \it name      & \it meta-type         & \it description \\ 

  \cdx{Type}    & $t \to prop$          & judgement form \\

  \cdx{Eqtype}  & $[t,t]\to prop$       & judgement form\\

  \cdx{Elem}    & $[i, t]\to prop$      & judgement form\\

  \cdx{Eqelem}  & $[i, i, t]\to prop$   & judgement form\\

  \cdx{Reduce}  & $[i, i]\to prop$      & extra judgement form\\[2ex]

  \cdx{N}       &     $t$               & natural numbers type\\

  \cdx{0}       &     $i$               & constructor\\

  \cdx{succ}    & $i\to i$              & constructor\\

  \cdx{rec}     & $[i,i,[i,i]\to i]\to i$       & eliminator\\[2ex]

  \cdx{Prod}    & $[t,i\to t]\to t$     & general product type\\

  \cdx{lambda}  & $(i\to i)\to i$       & constructor\\[2ex]

  \cdx{Sum}     & $[t, i\to t]\to t$    & general sum type\\

  \cdx{pair}    & $[i,i]\to i$          & constructor\\

  \cdx{split}   & $[i,[i,i]\to i]\to i$ & eliminator\\

  \cdx{fst} \cdx{snd} & $i\to i$        & projections\\[2ex]

  \cdx{inl} \cdx{inr} & $i\to i$        & constructors for $+$\\

  \cdx{when}    & $[i,i\to i, i\to i]\to i$    & eliminator for $+$\\[2ex]

  \cdx{Eq}      & $[t,i,i]\to t$        & equality type\\

  \cdx{eq}      & $i$                   & constructor\\[2ex]

  \cdx{F}       & $t$                   & empty type\\

  \cdx{contr}   & $i\to i$              & eliminator\\[2ex]

  \cdx{T}       & $t$                   & singleton type\\

  \cdx{tt}      & $i$                   & constructor

\end{tabular}

\end{center}

\caption{The constants of {\CTT}} \label{ctt-constants}

\end{figure}

{\CTT} supports all of Type Theory apart from list types, well-ordering

types, and universes.  Universes could be introduced {\em\`a la Tarski},

adding new constants as names for types.  The formulation {\em\`a la

  Russell}, where types denote themselves, is only possible if we identify

the meta-types~{\tt i} and~{\tt t}.  Most published formulations of

well-ordering types have difficulties involving extensionality of

functions; I suggest that you use some other method for defining recursive

types.  List types are easy to introduce by declaring new rules.

{\CTT} uses the 1982 version of Type Theory, with extensional equality.

The computation $a=b\in A$ and the equality $c\in Eq(A,a,b)$ are

interchangeable.  Its rewriting tactics prove theorems of the form $a=b\in

A$.  It could be modified to have intensional equality, but rewriting

tactics would have to prove theorems of the form $c\in Eq(A,a,b)$ and the

computation rules might require a separate simplifier.

\begin{figure} \tabcolsep=1em  %wider spacing in tables

\index{lambda abs@$\lambda$-abstractions!in \CTT}

\begin{center}

\begin{tabular}{llrrr} 

  \it symbol &\it name     &\it meta-type & \it priority & \it description \\

  \sdx{lam} & \cdx{lambda}  & $(i\To o)\To i$ & 10 & $\lambda$-abstraction

\end{tabular}

\end{center}

\subcaption{Binders} 

\begin{center}

\index{*"` symbol}\index{function applications!in \CTT}

\index{*"+ symbol}

\begin{tabular}{rrrr} 

  \it symbol & \it meta-type    & \it priority & \it description \\ 

  \tt `         & $[i,i]\to i$  & Left 55       & function application\\

  \tt +         & $[t,t]\to t$  & Right 30      & sum of two types

\end{tabular}

\end{center}

\subcaption{Infixes}

\index{*"* symbol}

\index{*"-"-"> symbol}

\begin{center} \tt\frenchspacing

\begin{tabular}{rrr} 

  \it external                  & \it internal  & \it standard notation \\ 

  \sdx{PROD} $x$:$A$ . $B[x]$   &  Prod($A$, $\lambda x.B[x]$) &

        \rm product $\prod@{x\in A}B[x]$ \\

  \sdx{SUM} $x$:$A$ . $B[x]$    & Sum($A$, $\lambda x.B[x]$) &

        \rm sum $\sum@{x\in A}B[x]$ \\

  $A$ --> $B$     &  Prod($A$, $\lambda x.B$) &

        \rm function space $A\to B$ \\

  $A$ * $B$       &  Sum($A$, $\lambda x.B$)  &

        \rm binary product $A\times B$

\end{tabular}

\end{center}

\subcaption{Translations} 

\index{*"= symbol}

\begin{center}

\dquotes

\[ \begin{array}{rcl}

prop    & = &  type " type"       \\

        & | & type " = " type     \\

        & | & term " : " type        \\

        & | & term " = " term " : " type 

\\[2ex]

type    & = & \hbox{expression of type~$t$} \\

        & | & "PROD~" id " : " type " . " type  \\

        & | & "SUM~~" id " : " type " . " type 

\\[2ex]

term    & = & \hbox{expression of type~$i$} \\

        & | & "lam " id~id^* " . " term   \\

        & | & "< " term " , " term " >"

\end{array} 

\]

\end{center}

\subcaption{Grammar}

\caption{Syntax of {\CTT}} \label{ctt-syntax}

\end{figure}

%%%%\section{Generic Packages}  typedsimp.ML????????????????

\section{Syntax}

The constants are shown in Fig.\ts\ref{ctt-constants}.  The infixes include

the function application operator (sometimes called `apply'), and the

2-place type operators.  Note that meta-level abstraction and application,

$\lambda x.b$ and $f(a)$, differ from object-level abstraction and

application, \hbox{\tt lam $x$.$b$} and $b{\tt`}a$.  A {\CTT}

function~$f$ is simply an individual as far as Isabelle is concerned: its

Isabelle type is~$i$, not say $i\To i$.

The notation for~{\CTT} (Fig.\ts\ref{ctt-syntax}) is based on that of

Nordstr\"om et al.~\cite{nordstrom90}.  The empty type is called $F$ and

the one-element type is $T$; other finite types are built as $T+T+T$, etc.

\index{*SUM symbol}\index{*PROD symbol}

Quantification is expressed using general sums $\sum@{x\in A}B[x]$ and

products $\prod@{x\in A}B[x]$.  Instead of {\tt Sum($A$,$B$)} and {\tt

  Prod($A$,$B$)} we may write \hbox{\tt SUM $x$:$A$.$B[x]$} and \hbox{\tt

  PROD $x$:$A$.$B[x]$}.  For example, we may write

\begin{ttbox}

SUM y:B. PROD x:A. C(x,y)   {\rm for}   Sum(B, \%y. Prod(A, \%x. C(x,y)))

\end{ttbox}

The special cases as \hbox{\tt$A$*$B$} and \hbox{\tt$A$-->$B$} abbreviate

general sums and products over a constant family.\footnote{Unlike normal

infix operators, {\tt*} and {\tt-->} merely define abbreviations; there are

no constants~{\tt op~*} and~\hbox{\tt op~-->}.}  Isabelle accepts these

abbreviations in parsing and uses them whenever possible for printing.

\begin{figure} 

\begin{ttbox}

\tdx{refl_type}         A type ==> A = A

\tdx{refl_elem}         a : A ==> a = a : A

\tdx{sym_type}          A = B ==> B = A

\tdx{sym_elem}          a = b : A ==> b = a : A

\tdx{trans_type}        [| A = B;  B = C |] ==> A = C

\tdx{trans_elem}        [| a = b : A;  b = c : A |] ==> a = c : A

\tdx{equal_types}       [| a : A;  A = B |] ==> a : B

\tdx{equal_typesL}      [| a = b : A;  A = B |] ==> a = b : B

\tdx{subst_type}        [| a : A;  !!z. z:A ==> B(z) type |] ==> B(a) type

\tdx{subst_typeL}       [| a = c : A;  !!z. z:A ==> B(z) = D(z) 

                  |] ==> B(a) = D(c)

\tdx{subst_elem}        [| a : A;  !!z. z:A ==> b(z):B(z) |] ==> b(a):B(a)

\tdx{subst_elemL}       [| a = c : A;  !!z. z:A ==> b(z) = d(z) : B(z) 

                  |] ==> b(a) = d(c) : B(a)

\tdx{refl_red}          Reduce(a,a)

\tdx{red_if_equal}      a = b : A ==> Reduce(a,b)

\tdx{trans_red}         [| a = b : A;  Reduce(b,c) |] ==> a = c : A

\end{ttbox}

\caption{General equality rules} \label{ctt-equality}

\end{figure}

\begin{figure} 

\begin{ttbox}

\tdx{NF}        N type

\tdx{NI0}       0 : N

\tdx{NI_succ}   a : N ==> succ(a) : N

\tdx{NI_succL}  a = b : N ==> succ(a) = succ(b) : N

\tdx{NE}        [| p: N;  a: C(0);  

             !!u v. [| u: N; v: C(u) |] ==> b(u,v): C(succ(u)) 

          |] ==> rec(p, a, \%u v.b(u,v)) : C(p)

\tdx{NEL}       [| p = q : N;  a = c : C(0);  

             !!u v. [| u: N; v: C(u) |] ==> b(u,v)=d(u,v): C(succ(u))

          |] ==> rec(p, a, \%u v.b(u,v)) = rec(q,c,d) : C(p)

\tdx{NC0}       [| a: C(0);  

             !!u v. [| u: N; v: C(u) |] ==> b(u,v): C(succ(u))

          |] ==> rec(0, a, \%u v.b(u,v)) = a : C(0)

\tdx{NC_succ}   [| p: N;  a: C(0);  

             !!u v. [| u: N; v: C(u) |] ==> b(u,v): C(succ(u)) 

          |] ==> rec(succ(p), a, \%u v.b(u,v)) =

                 b(p, rec(p, a, \%u v.b(u,v))) : C(succ(p))

\tdx{zero_ne_succ}      [| a: N;  0 = succ(a) : N |] ==> 0: F

\end{ttbox}

\caption{Rules for type~$N$} \label{ctt-N}

\end{figure}

\begin{figure} 

\begin{ttbox}

\tdx{ProdF}     [| A type; !!x. x:A ==> B(x) type |] ==> PROD x:A.B(x) type

\tdx{ProdFL}    [| A = C;  !!x. x:A ==> B(x) = D(x) |] ==> 

          PROD x:A.B(x) = PROD x:C.D(x)

\tdx{ProdI}     [| A type;  !!x. x:A ==> b(x):B(x)

          |] ==> lam x.b(x) : PROD x:A.B(x)

\tdx{ProdIL}    [| A type;  !!x. x:A ==> b(x) = c(x) : B(x)

          |] ==> lam x.b(x) = lam x.c(x) : PROD x:A.B(x)

\tdx{ProdE}     [| p : PROD x:A.B(x);  a : A |] ==> p`a : B(a)

\tdx{ProdEL}    [| p=q: PROD x:A.B(x);  a=b : A |] ==> p`a = q`b : B(a)

\tdx{ProdC}     [| a : A;  !!x. x:A ==> b(x) : B(x)

          |] ==> (lam x.b(x)) ` a = b(a) : B(a)

\tdx{ProdC2}    p : PROD x:A.B(x) ==> (lam x. p`x) = p : PROD x:A.B(x)

\end{ttbox}

\caption{Rules for the product type $\prod\sb{x\in A}B[x]$} \label{ctt-prod}

\end{figure}

\begin{figure} 

\begin{ttbox}

\tdx{SumF}      [| A type;  !!x. x:A ==> B(x) type |] ==> SUM x:A.B(x) type

\tdx{SumFL}     [| A = C;  !!x. x:A ==> B(x) = D(x) 

          |] ==> SUM x:A.B(x) = SUM x:C.D(x)

\tdx{SumI}      [| a : A;  b : B(a) |] ==> <a,b> : SUM x:A.B(x)

\tdx{SumIL}     [| a=c:A;  b=d:B(a) |] ==> <a,b> = <c,d> : SUM x:A.B(x)

\tdx{SumE}      [| p: SUM x:A.B(x);  

             !!x y. [| x:A; y:B(x) |] ==> c(x,y): C(<x,y>) 

          |] ==> split(p, \%x y.c(x,y)) : C(p)

\tdx{SumEL}     [| p=q : SUM x:A.B(x); 

             !!x y. [| x:A; y:B(x) |] ==> c(x,y)=d(x,y): C(<x,y>)

          |] ==> split(p, \%x y.c(x,y)) = split(q, \%x y.d(x,y)) : C(p)

\tdx{SumC}      [| a: A;  b: B(a);

             !!x y. [| x:A; y:B(x) |] ==> c(x,y): C(<x,y>)

          |] ==> split(<a,b>, \%x y.c(x,y)) = c(a,b) : C(<a,b>)

\tdx{fst_def}   fst(a) == split(a, \%x y.x)

\tdx{snd_def}   snd(a) == split(a, \%x y.y)

\end{ttbox}

\caption{Rules for the sum type $\sum\sb{x\in A}B[x]$} \label{ctt-sum}

\end{figure}

\begin{figure} 

\begin{ttbox}

\tdx{PlusF}       [| A type;  B type |] ==> A+B type

\tdx{PlusFL}      [| A = C;  B = D |] ==> A+B = C+D

\tdx{PlusI_inl}   [| a : A;  B type |] ==> inl(a) : A+B

\tdx{PlusI_inlL}  [| a = c : A;  B type |] ==> inl(a) = inl(c) : A+B

\tdx{PlusI_inr}   [| A type;  b : B |] ==> inr(b) : A+B

\tdx{PlusI_inrL}  [| A type;  b = d : B |] ==> inr(b) = inr(d) : A+B

\tdx{PlusE}     [| p: A+B;

             !!x. x:A ==> c(x): C(inl(x));  

             !!y. y:B ==> d(y): C(inr(y))

          |] ==> when(p, \%x.c(x), \%y.d(y)) : C(p)

\tdx{PlusEL}    [| p = q : A+B;

             !!x. x: A ==> c(x) = e(x) : C(inl(x));   

             !!y. y: B ==> d(y) = f(y) : C(inr(y))

          |] ==> when(p, \%x.c(x), \%y.d(y)) = 

                 when(q, \%x.e(x), \%y.f(y)) : C(p)

\tdx{PlusC_inl} [| a: A;

             !!x. x:A ==> c(x): C(inl(x));  

             !!y. y:B ==> d(y): C(inr(y))

          |] ==> when(inl(a), \%x.c(x), \%y.d(y)) = c(a) : C(inl(a))

\tdx{PlusC_inr} [| b: B;

             !!x. x:A ==> c(x): C(inl(x));  

             !!y. y:B ==> d(y): C(inr(y))

          |] ==> when(inr(b), \%x.c(x), \%y.d(y)) = d(b) : C(inr(b))

\end{ttbox}

\caption{Rules for the binary sum type $A+B$} \label{ctt-plus}

\end{figure}

\begin{figure} 

\begin{ttbox}

\tdx{FF}        F type

\tdx{FE}        [| p: F;  C type |] ==> contr(p) : C

\tdx{FEL}       [| p = q : F;  C type |] ==> contr(p) = contr(q) : C

\tdx{TF}        T type

\tdx{TI}        tt : T

\tdx{TE}        [| p : T;  c : C(tt) |] ==> c : C(p)

\tdx{TEL}       [| p = q : T;  c = d : C(tt) |] ==> c = d : C(p)

\tdx{TC}        p : T ==> p = tt : T)

\end{ttbox}

\caption{Rules for types $F$ and $T$} \label{ctt-ft}

\end{figure}

\begin{figure} 

\begin{ttbox}

\tdx{EqF}       [| A type;  a : A;  b : A |] ==> Eq(A,a,b) type

\tdx{EqFL}      [| A=B;  a=c: A;  b=d : A |] ==> Eq(A,a,b) = Eq(B,c,d)

\tdx{EqI}       a = b : A ==> eq : Eq(A,a,b)

\tdx{EqE}       p : Eq(A,a,b) ==> a = b : A

\tdx{EqC}       p : Eq(A,a,b) ==> p = eq : Eq(A,a,b)

\end{ttbox}

\caption{Rules for the equality type $Eq(A,a,b)$} \label{ctt-eq}

\end{figure}

\begin{figure} 

\begin{ttbox}

\tdx{replace_type}    [| B = A;  a : A |] ==> a : B

\tdx{subst_eqtyparg}  [| a=c : A;  !!z. z:A ==> B(z) type |] ==> B(a)=B(c)

\tdx{subst_prodE}     [| p: Prod(A,B);  a: A;  !!z. z: B(a) ==> c(z): C(z)

                |] ==> c(p`a): C(p`a)

\tdx{SumIL2}    [| c=a : A;  d=b : B(a) |] ==> <c,d> = <a,b> : Sum(A,B)

\tdx{SumE_fst}  p : Sum(A,B) ==> fst(p) : A

\tdx{SumE_snd}  [| p: Sum(A,B);  A type;  !!x. x:A ==> B(x) type

          |] ==> snd(p) : B(fst(p))

\end{ttbox}

\caption{Derived rules for {\CTT}} \label{ctt-derived}

\end{figure}

\section{Rules of inference}

The rules obey the following naming conventions.  Type formation rules have

the suffix~{\tt F}\@.  Introduction rules have the suffix~{\tt I}\@.

Elimination rules have the suffix~{\tt E}\@.  Computation rules, which

describe the reduction of eliminators, have the suffix~{\tt C}\@.  The

equality versions of the rules (which permit reductions on subterms) are

called {\bf long} rules; their names have the suffix~{\tt L}\@.

Introduction and computation rules are often further suffixed with

constructor names.

Figure~\ref{ctt-equality} presents the equality rules.  Most of them are

straightforward: reflexivity, symmetry, transitivity and substitution.  The

judgement \cdx{Reduce} does not belong to Type Theory proper; it has

been added to implement rewriting.  The judgement ${\tt Reduce}(a,b)$ holds

when $a=b:A$ holds.  It also holds when $a$ and $b$ are syntactically

identical, even if they are ill-typed, because rule {\tt refl_red} does

not verify that $a$ belongs to $A$.  

The {\tt Reduce} rules do not give rise to new theorems about the standard

judgements.  The only rule with {\tt Reduce} in a premise is

{\tt trans_red}, whose other premise ensures that $a$ and~$b$ (and thus

$c$) are well-typed.

Figure~\ref{ctt-N} presents the rules for~$N$, the type of natural numbers.

They include \tdx{zero_ne_succ}, which asserts $0\not=n+1$.  This is

the fourth Peano axiom and cannot be derived without universes \cite[page

91]{martinlof84}.  

The constant \cdx{rec} constructs proof terms when mathematical

induction, rule~\tdx{NE}, is applied.  It can also express primitive

recursion.  Since \cdx{rec} can be applied to higher-order functions,

it can even express Ackermann's function, which is not primitive recursive

\cite[page~104]{thompson91}.

Figure~\ref{ctt-prod} shows the rules for general product types, which

include function types as a special case.  The rules correspond to the

predicate calculus rules for universal quantifiers and implication.  They

also permit reasoning about functions, with the rules of a typed

$\lambda$-calculus.

Figure~\ref{ctt-sum} shows the rules for general sum types, which

include binary product types as a special case.  The rules correspond to the

predicate calculus rules for existential quantifiers and conjunction.  They

also permit reasoning about ordered pairs, with the projections

\cdx{fst} and~\cdx{snd}.

Figure~\ref{ctt-plus} shows the rules for binary sum types.  They

correspond to the predicate calculus rules for disjunction.  They also

permit reasoning about disjoint sums, with the injections \cdx{inl}

and~\cdx{inr} and case analysis operator~\cdx{when}.

Figure~\ref{ctt-ft} shows the rules for the empty and unit types, $F$

and~$T$.  They correspond to the predicate calculus rules for absurdity and

truth.

Figure~\ref{ctt-eq} shows the rules for equality types.  If $a=b\in A$ is

provable then \cdx{eq} is a canonical element of the type $Eq(A,a,b)$,

and vice versa.  These rules define extensional equality; the most recent

versions of Type Theory use intensional equality~\cite{nordstrom90}.

Figure~\ref{ctt-derived} presents the derived rules.  The rule

\tdx{subst_prodE} is derived from {\tt prodE}, and is easier to use

in backwards proof.  The rules \tdx{SumE_fst} and \tdx{SumE_snd}

express the typing of~\cdx{fst} and~\cdx{snd}; together, they are

roughly equivalent to~{\tt SumE} with the advantage of creating no

parameters.  Section~\ref{ctt-choice} below demonstrates these rules in a

proof of the Axiom of Choice.

All the rules are given in $\eta$-expanded form.  For instance, every

occurrence of $\lambda u\,v.b(u,v)$ could be abbreviated to~$b$ in the

rules for~$N$.  The expanded form permits Isabelle to preserve bound

variable names during backward proof.  Names of bound variables in the

conclusion (here, $u$ and~$v$) are matched with corresponding bound

variables in the premises.

\section{Rule lists}

The Type Theory tactics provide rewriting, type inference, and logical

reasoning.  Many proof procedures work by repeatedly resolving certain Type

Theory rules against a proof state.  {\CTT} defines lists --- each with

type

\hbox{\tt thm list} --- of related rules. 

\begin{ttdescription}

\item[\ttindexbold{form_rls}] 

contains formation rules for the types $N$, $\Pi$, $\Sigma$, $+$, $Eq$,

$F$, and $T$.

\item[\ttindexbold{formL_rls}] 

contains long formation rules for $\Pi$, $\Sigma$, $+$, and $Eq$.  (For

other types use \tdx{refl_type}.)

\item[\ttindexbold{intr_rls}] 

contains introduction rules for the types $N$, $\Pi$, $\Sigma$, $+$, and

$T$.

\item[\ttindexbold{intrL_rls}] 

contains long introduction rules for $N$, $\Pi$, $\Sigma$, and $+$.  (For

$T$ use \tdx{refl_elem}.)

\item[\ttindexbold{elim_rls}] 

contains elimination rules for the types $N$, $\Pi$, $\Sigma$, $+$, and

$F$.  The rules for $Eq$ and $T$ are omitted because they involve no

eliminator.

\item[\ttindexbold{elimL_rls}] 

contains long elimination rules for $N$, $\Pi$, $\Sigma$, $+$, and $F$.

\item[\ttindexbold{comp_rls}] 

contains computation rules for the types $N$, $\Pi$, $\Sigma$, and $+$.

Those for $Eq$ and $T$ involve no eliminator.

\item[\ttindexbold{basic_defs}] 

contains the definitions of {\tt fst} and {\tt snd}.  

\end{ttdescription}

\section{Tactics for subgoal reordering}

\begin{ttbox}

test_assume_tac : int -> tactic

typechk_tac     : thm list -> tactic

equal_tac       : thm list -> tactic

intr_tac        : thm list -> tactic

\end{ttbox}

Blind application of {\CTT} rules seldom leads to a proof.  The elimination

rules, especially, create subgoals containing new unknowns.  These subgoals

unify with anything, creating a huge search space.  The standard tactic

\ttindex{filt_resolve_tac} 

(see \iflabelundefined{filt_resolve_tac}{the {\em Reference Manual\/}}%

        {\S\ref{filt_resolve_tac}})

%

fails for goals that are too flexible; so does the {\CTT} tactic {\tt

  test_assume_tac}.  Used with the tactical \ttindex{REPEAT_FIRST} they

achieve a simple kind of subgoal reordering: the less flexible subgoals are

attempted first.  Do some single step proofs, or study the examples below,

to see why this is necessary.

\begin{ttdescription}

\item[\ttindexbold{test_assume_tac} $i$] 

uses {\tt assume_tac} to solve the subgoal by assumption, but only if

subgoal~$i$ has the form $a\in A$ and the head of $a$ is not an unknown.

Otherwise, it fails.

\item[\ttindexbold{typechk_tac} $thms$] 

uses $thms$ with formation, introduction, and elimination rules to check

the typing of constructions.  It is designed to solve goals of the form

$a\in \Var{A}$, where $a$ is rigid and $\Var{A}$ is flexible; thus it

performs type inference.  The tactic can also solve goals of

the form $A\;\rm type$.

\item[\ttindexbold{equal_tac} $thms$]

uses $thms$ with the long introduction and elimination rules to solve goals

of the form $a=b\in A$, where $a$ is rigid.  It is intended for deriving

the long rules for defined constants such as the arithmetic operators.  The

tactic can also perform type checking.

\item[\ttindexbold{intr_tac} $thms$]

uses $thms$ with the introduction rules to break down a type.  It is

designed for goals like $\Var{a}\in A$ where $\Var{a}$ is flexible and $A$

rigid.  These typically arise when trying to prove a proposition~$A$,

expressed as a type.

\end{ttdescription}

\section{Rewriting tactics}

\begin{ttbox}

rew_tac     : thm list -> tactic

hyp_rew_tac : thm list -> tactic

\end{ttbox}

Object-level simplification is accomplished through proof, using the {\tt

  CTT} equality rules and the built-in rewriting functor

{\tt TSimpFun}.%

\footnote{This should not be confused with Isabelle's main simplifier; {\tt

    TSimpFun} is only useful for {\CTT} and similar logics with type

  inference rules.  At present it is undocumented.} 

%

The rewrites include the computation rules and other equations.  The long

versions of the other rules permit rewriting of subterms and subtypes.

Also used are transitivity and the extra judgement form \cdx{Reduce}.

Meta-level simplification handles only definitional equality.

\begin{ttdescription}

\item[\ttindexbold{rew_tac} $thms$]

applies $thms$ and the computation rules as left-to-right rewrites.  It

solves the goal $a=b\in A$ by rewriting $a$ to $b$.  If $b$ is an unknown

then it is assigned the rewritten form of~$a$.  All subgoals are rewritten.

\item[\ttindexbold{hyp_rew_tac} $thms$]

is like {\tt rew_tac}, but includes as rewrites any equations present in

the assumptions.

\end{ttdescription}

\section{Tactics for logical reasoning}

Interpreting propositions as types lets {\CTT} express statements of

intuitionistic logic.  However, Constructive Type Theory is not just

another syntax for first-order logic.  There are fundamental differences.

\index{assumptions!in {\CTT}}

Can assumptions be deleted after use?  Not every occurrence of a type

represents a proposition, and Type Theory assumptions declare variables.

In first-order logic, $\disj$-elimination with the assumption $P\disj Q$

creates one subgoal assuming $P$ and another assuming $Q$, and $P\disj Q$

can be deleted safely.  In Type Theory, $+$-elimination with the assumption

$z\in A+B$ creates one subgoal assuming $x\in A$ and another assuming $y\in

B$ (for arbitrary $x$ and $y$).  Deleting $z\in A+B$ when other assumptions

refer to $z$ may render the subgoal unprovable: arguably,

meaningless.

Isabelle provides several tactics for predicate calculus reasoning in \CTT:

\begin{ttbox}

mp_tac       : int -> tactic

add_mp_tac   : int -> tactic

safestep_tac : thm list -> int -> tactic

safe_tac     : thm list -> int -> tactic

step_tac     : thm list -> int -> tactic

pc_tac       : thm list -> int -> tactic

\end{ttbox}

These are loosely based on the intuitionistic proof procedures

of~\thydx{FOL}.  For the reasons discussed above, a rule that is safe for

propositional reasoning may be unsafe for type checking; thus, some of the

`safe' tactics are misnamed.

\begin{ttdescription}

\item[\ttindexbold{mp_tac} $i$] 

searches in subgoal~$i$ for assumptions of the form $f\in\Pi(A,B)$ and

$a\in A$, where~$A$ may be found by unification.  It replaces

$f\in\Pi(A,B)$ by $z\in B(a)$, where~$z$ is a new parameter.  The tactic

can produce multiple outcomes for each suitable pair of assumptions.  In

short, {\tt mp_tac} performs Modus Ponens among the assumptions.

\item[\ttindexbold{add_mp_tac} $i$]

is like {\tt mp_tac}~$i$ but retains the assumption $f\in\Pi(A,B)$.  It

avoids information loss but obviously loops if repeated.

\item[\ttindexbold{safestep_tac} $thms$ $i$]

attacks subgoal~$i$ using formation rules and certain other `safe' rules

(\tdx{FE}, \tdx{ProdI}, \tdx{SumE}, \tdx{PlusE}), calling

{\tt mp_tac} when appropriate.  It also uses~$thms$,

which are typically premises of the rule being derived.

\item[\ttindexbold{safe_tac} $thms$ $i$] attempts to solve subgoal~$i$ by

  means of backtracking, using {\tt safestep_tac}.

\item[\ttindexbold{step_tac} $thms$ $i$]

tries to reduce subgoal~$i$ using {\tt safestep_tac}, then tries unsafe

rules.  It may produce multiple outcomes.

\item[\ttindexbold{pc_tac} $thms$ $i$]

tries to solve subgoal~$i$ by backtracking, using {\tt step_tac}.

\end{ttdescription}

\begin{figure} 

\index{#+@{\tt\#+} symbol}

\index{*"- symbol}

\index{*"|"-"| symbol}

\index{#*@{\tt\#*} symbol}

\index{*div symbol}

\index{*mod symbol}

\begin{constants}

  \it symbol  & \it meta-type & \it priority & \it description \\ 

  \tt \#*       & $[i,i]\To i$  &  Left 70      & multiplication \\

  \tt div       & $[i,i]\To i$  &  Left 70      & division\\

  \tt mod       & $[i,i]\To i$  &  Left 70      & modulus\\

  \tt \#+       & $[i,i]\To i$  &  Left 65      & addition\\

  \tt -         & $[i,i]\To i$  &  Left 65      & subtraction\\

  \verb'|-|'    & $[i,i]\To i$  &  Left 65      & absolute difference

\end{constants}

\begin{ttbox}

\tdx{add_def}           a#+b  == rec(a, b, \%u v.succ(v))  

\tdx{diff_def}          a-b   == rec(b, a, \%u v.rec(v, 0, \%x y.x))  

\tdx{absdiff_def}       a|-|b == (a-b) #+ (b-a)  

\tdx{mult_def}          a#*b  == rec(a, 0, \%u v. b #+ v)  

\tdx{mod_def}           a mod b ==

                  rec(a, 0, \%u v. rec(succ(v) |-| b, 0, \%x y.succ(v)))

\tdx{div_def}           a div b ==

                  rec(a, 0, \%u v. rec(succ(u) mod b, succ(v), \%x y.v))

\tdx{add_typing}        [| a:N;  b:N |] ==> a #+ b : N

\tdx{addC0}             b:N ==> 0 #+ b = b : N

\tdx{addC_succ}         [| a:N;  b:N |] ==> succ(a) #+ b = succ(a #+ b) : N

\tdx{add_assoc}         [| a:N;  b:N;  c:N |] ==> 

                  (a #+ b) #+ c = a #+ (b #+ c) : N

\tdx{add_commute}       [| a:N;  b:N |] ==> a #+ b = b #+ a : N

\tdx{mult_typing}       [| a:N;  b:N |] ==> a #* b : N

\tdx{multC0}            b:N ==> 0 #* b = 0 : N

\tdx{multC_succ}        [| a:N;  b:N |] ==> succ(a) #* b = b #+ (a#*b) : N

\tdx{mult_commute}      [| a:N;  b:N |] ==> a #* b = b #* a : N

\tdx{add_mult_dist}     [| a:N;  b:N;  c:N |] ==> 

                  (a #+ b) #* c = (a #* c) #+ (b #* c) : N

\tdx{mult_assoc}        [| a:N;  b:N;  c:N |] ==> 

                  (a #* b) #* c = a #* (b #* c) : N

\tdx{diff_typing}       [| a:N;  b:N |] ==> a - b : N

\tdx{diffC0}            a:N ==> a - 0 = a : N

\tdx{diff_0_eq_0}       b:N ==> 0 - b = 0 : N

\tdx{diff_succ_succ}    [| a:N;  b:N |] ==> succ(a) - succ(b) = a - b : N

\tdx{diff_self_eq_0}    a:N ==> a - a = 0 : N

\tdx{add_inverse_diff}  [| a:N;  b:N;  b-a=0 : N |] ==> b #+ (a-b) = a : N

\caption{The theory of arithmetic} \label{ctt_arith}

\end{ttbox}

\end{figure}

\section{A theory of arithmetic}

\thydx{Arith} is a theory of elementary arithmetic.  It proves the

properties of addition, multiplication, subtraction, division, and

remainder, culminating in the theorem

\[ a \bmod b + (a/b)\times b = a. \]

Figure~\ref{ctt_arith} presents the definitions and some of the key

theorems, including commutative, distributive, and associative laws.

The operators~\verb'#+', \verb'-', \verb'|-|', \verb'#*', \verb'mod'

and~\verb'div' stand for sum, difference, absolute difference, product,

remainder and quotient, respectively.  Since Type Theory has only primitive

recursion, some of their definitions may be obscure.  

The difference~$a-b$ is computed by taking $b$ predecessors of~$a$, where

the predecessor function is $\lambda v. {\tt rec}(v, 0, \lambda x\,y.x)$.

The remainder $a\bmod b$ counts up to~$a$ in a cyclic fashion, using 0

as the successor of~$b-1$.  Absolute difference is used to test the

equality $succ(v)=b$.

The quotient $a/b$ is computed by adding one for every number $x$

such that $0\leq x \leq a$ and $x\bmod b = 0$.

\section{The examples directory}

This directory contains examples and experimental proofs in {\CTT}.

\begin{ttdescription}

\item[CTT/ex/typechk.ML]

contains simple examples of type checking and type deduction.

\item[CTT/ex/elim.ML]

contains some examples from Martin-L\"of~\cite{martinlof84}, proved using 

{\tt pc_tac}.

\item[CTT/ex/equal.ML]

contains simple examples of rewriting.

\item[CTT/ex/synth.ML]

demonstrates the use of unknowns with some trivial examples of program

synthesis. 

\end{ttdescription}

\section{Example: type inference}

Type inference involves proving a goal of the form $a\in\Var{A}$, where $a$

is a term and $\Var{A}$ is an unknown standing for its type.  The type,

initially

unknown, takes shape in the course of the proof.  Our example is the

predecessor function on the natural numbers.

\begin{ttbox}

goal CTT.thy "lam n. rec(n, 0, \%x y.x) : ?A";

{\out Level 0}

{\out lam n. rec(n,0,\%x y. x) : ?A}

{\out  1. lam n. rec(n,0,\%x y. x) : ?A}

\end{ttbox}

Since the term is a Constructive Type Theory $\lambda$-abstraction (not to

be confused with a meta-level abstraction), we apply the rule

\tdx{ProdI}, for $\Pi$-introduction.  This instantiates~$\Var{A}$ to a

product type of unknown domain and range.

\begin{ttbox}

by (resolve_tac [ProdI] 1);

{\out Level 1}

{\out lam n. rec(n,0,\%x y. x) : PROD x:?A1. ?B1(x)}

{\out  1. ?A1 type}

{\out  2. !!n. n : ?A1 ==> rec(n,0,\%x y. x) : ?B1(n)}

\end{ttbox}

Subgoal~1 is too flexible.  It can be solved by instantiating~$\Var{A@1}$

to any type, but most instantiations will invalidate subgoal~2.  We

therefore tackle the latter subgoal.  It asks the type of a term beginning

with {\tt rec}, which can be found by $N$-elimination.%

\index{*NE theorem}

\begin{ttbox}

by (eresolve_tac [NE] 2);

{\out Level 2}

{\out lam n. rec(n,0,\%x y. x) : PROD x:N. ?C2(x,x)}

{\out  1. N type}

{\out  2. !!n. 0 : ?C2(n,0)}

{\out  3. !!n x y. [| x : N; y : ?C2(n,x) |] ==> x : ?C2(n,succ(x))}

\end{ttbox}

Subgoal~1 is no longer flexible: we now know~$\Var{A@1}$ is the type of

natural numbers.  However, let us continue proving nontrivial subgoals.

Subgoal~2 asks, what is the type of~0?\index{*NIO theorem}

\begin{ttbox}

by (resolve_tac [NI0] 2);

{\out Level 3}

{\out lam n. rec(n,0,\%x y. x) : N --> N}

{\out  1. N type}

{\out  2. !!n x y. [| x : N; y : N |] ==> x : N}

\end{ttbox}

The type~$\Var{A}$ is now fully determined.  It is the product type

$\prod@{x\in N}N$, which is shown as the function type $N\to N$ because

there is no dependence on~$x$.  But we must prove all the subgoals to show

that the original term is validly typed.  Subgoal~2 is provable by

assumption and the remaining subgoal falls by $N$-formation.%

\index{*NF theorem}

\begin{ttbox}

by (assume_tac 2);

{\out Level 4}

{\out lam n. rec(n,0,\%x y. x) : N --> N}

{\out  1. N type}

\ttbreak

by (resolve_tac [NF] 1);

{\out Level 5}

{\out lam n. rec(n,0,\%x y. x) : N --> N}

{\out No subgoals!}

\end{ttbox}

Calling \ttindex{typechk_tac} can prove this theorem in one step.

Even if the original term is ill-typed, one can infer a type for it, but

unprovable subgoals will be left.  As an exercise, try to prove the

following invalid goal:

\begin{ttbox}

goal CTT.thy "lam n. rec(n, 0, \%x y.tt) : ?A";

\end{ttbox}

\section{An example of logical reasoning}

Logical reasoning in Type Theory involves proving a goal of the form

$\Var{a}\in A$, where type $A$ expresses a proposition and $\Var{a}$ stands

for its proof term, a value of type $A$.  The proof term is initially

unknown and takes shape during the proof.  

Our example expresses a theorem about quantifiers in a sorted logic:

\[ \infer{(\ex{x\in A}P(x)) \disj (\ex{x\in A}Q(x))}

         {\ex{x\in A}P(x)\disj Q(x)} 

\]

By the propositions-as-types principle, this is encoded

using~$\Sigma$ and~$+$ types.  A special case of it expresses a

distributive law of Type Theory: 

\[ \infer{(A\times B) + (A\times C)}{A\times(B+C)} \]

Generalizing this from $\times$ to $\Sigma$, and making the typing

conditions explicit, yields the rule we must derive:

\[ \infer{\Var{a} \in (\sum@{x\in A} B(x)) + (\sum@{x\in A} C(x))}

         {\hbox{$A$ type} &

          \infer*{\hbox{$B(x)$ type}}{[x\in A]}  &

          \infer*{\hbox{$C(x)$ type}}{[x\in A]}  &

          p\in \sum@{x\in A} B(x)+C(x)} 

\]

To begin, we bind the rule's premises --- returned by the~{\tt goal}

command --- to the {\ML} variable~{\tt prems}.

\begin{ttbox}

val prems = goal CTT.thy

    "[| A type;                       \ttback

\ttback       !!x. x:A ==> B(x) type;       \ttback

\ttback       !!x. x:A ==> C(x) type;       \ttback

\ttback       p: SUM x:A. B(x) + C(x)       \ttback

\ttback    |] ==>  ?a : (SUM x:A. B(x)) + (SUM x:A. C(x))";

{\out Level 0}

{\out ?a : (SUM x:A. B(x)) + (SUM x:A. C(x))}

{\out  1. ?a : (SUM x:A. B(x)) + (SUM x:A. C(x))}

\ttbreak

{\out val prems = ["A type  [A type]",}

{\out              "?x : A ==> B(?x) type  [!!x. x : A ==> B(x) type]",}

{\out              "?x : A ==> C(?x) type  [!!x. x : A ==> C(x) type]",}

{\out              "p : SUM x:A. B(x) + C(x)  [p : SUM x:A. B(x) + C(x)]"]}

{\out             : thm list}

\end{ttbox}

The last premise involves the sum type~$\Sigma$.  Since it is a premise

rather than the assumption of a goal, it cannot be found by {\tt

  eresolve_tac}.  We could insert it (and the other atomic premise) by

calling

\begin{ttbox}

cut_facts_tac prems 1;

\end{ttbox}

A forward proof step is more straightforward here.  Let us resolve the

$\Sigma$-elimination rule with the premises using~\ttindex{RL}.  This

inference yields one result, which we supply to {\tt

  resolve_tac}.\index{*SumE theorem}

\begin{ttbox}

by (resolve_tac (prems RL [SumE]) 1);

{\out Level 1}

{\out split(p,?c1) : (SUM x:A. B(x)) + (SUM x:A. C(x))}

{\out  1. !!x y.}

{\out        [| x : A; y : B(x) + C(x) |] ==>}

{\out        ?c1(x,y) : (SUM x:A. B(x)) + (SUM x:A. C(x))}

\end{ttbox}

The subgoal has two new parameters, $x$ and~$y$.  In the main goal,

$\Var{a}$ has been instantiated with a \cdx{split} term.  The

assumption $y\in B(x) + C(x)$ is eliminated next, causing a case split and

creating the parameter~$xa$.  This inference also inserts~\cdx{when}

into the main goal.\index{*PlusE theorem}

\begin{ttbox}

by (eresolve_tac [PlusE] 1);

{\out Level 2}

{\out split(p,\%x y. when(y,?c2(x,y),?d2(x,y)))}

{\out : (SUM x:A. B(x)) + (SUM x:A. C(x))}

{\out  1. !!x y xa.}

{\out        [| x : A; xa : B(x) |] ==>}

{\out        ?c2(x,y,xa) : (SUM x:A. B(x)) + (SUM x:A. C(x))}

\ttbreak

{\out  2. !!x y ya.}

{\out        [| x : A; ya : C(x) |] ==>}

{\out        ?d2(x,y,ya) : (SUM x:A. B(x)) + (SUM x:A. C(x))}

\end{ttbox}

To complete the proof object for the main goal, we need to instantiate the

terms $\Var{c@2}(x,y,xa)$ and $\Var{d@2}(x,y,xa)$.  We attack subgoal~1 by

a~$+$-introduction rule; since the goal assumes $xa\in B(x)$, we take the left

injection~(\cdx{inl}).

\index{*PlusI_inl theorem}

\begin{ttbox}

by (resolve_tac [PlusI_inl] 1);

{\out Level 3}

{\out split(p,\%x y. when(y,\%xa. inl(?a3(x,y,xa)),?d2(x,y)))}

{\out : (SUM x:A. B(x)) + (SUM x:A. C(x))}

{\out  1. !!x y xa. [| x : A; xa : B(x) |] ==> ?a3(x,y,xa) : SUM x:A. B(x)}

{\out  2. !!x y xa. [| x : A; xa : B(x) |] ==> SUM x:A. C(x) type}

\ttbreak

{\out  3. !!x y ya.}

{\out        [| x : A; ya : C(x) |] ==>}

{\out        ?d2(x,y,ya) : (SUM x:A. B(x)) + (SUM x:A. C(x))}

\end{ttbox}

A new subgoal~2 has appeared, to verify that $\sum@{x\in A}C(x)$ is a type.

Continuing to work on subgoal~1, we apply the $\Sigma$-introduction rule.

This instantiates the term $\Var{a@3}(x,y,xa)$; the main goal now contains

an ordered pair, whose components are two new unknowns.%

\index{*SumI theorem}

\begin{ttbox}

by (resolve_tac [SumI] 1);

{\out Level 4}

{\out split(p,\%x y. when(y,\%xa. inl(<?a4(x,y,xa),?b4(x,y,xa)>),?d2(x,y)))}

{\out : (SUM x:A. B(x)) + (SUM x:A. C(x))}

{\out  1. !!x y xa. [| x : A; xa : B(x) |] ==> ?a4(x,y,xa) : A}

{\out  2. !!x y xa. [| x : A; xa : B(x) |] ==> ?b4(x,y,xa) : B(?a4(x,y,xa))}

{\out  3. !!x y xa. [| x : A; xa : B(x) |] ==> SUM x:A. C(x) type}

{\out  4. !!x y ya.}

{\out        [| x : A; ya : C(x) |] ==>}

{\out        ?d2(x,y,ya) : (SUM x:A. B(x)) + (SUM x:A. C(x))}

\end{ttbox}

The two new subgoals both hold by assumption.  Observe how the unknowns

$\Var{a@4}$ and $\Var{b@4}$ are instantiated throughout the proof state.

\begin{ttbox}

by (assume_tac 1);

{\out Level 5}

{\out split(p,\%x y. when(y,\%xa. inl(<x,?b4(x,y,xa)>),?d2(x,y)))}

{\out : (SUM x:A. B(x)) + (SUM x:A. C(x))}

\ttbreak

{\out  1. !!x y xa. [| x : A; xa : B(x) |] ==> ?b4(x,y,xa) : B(x)}

{\out  2. !!x y xa. [| x : A; xa : B(x) |] ==> SUM x:A. C(x) type}

{\out  3. !!x y ya.}

{\out        [| x : A; ya : C(x) |] ==>}

{\out        ?d2(x,y,ya) : (SUM x:A. B(x)) + (SUM x:A. C(x))}

\ttbreak

by (assume_tac 1);

{\out Level 6}

{\out split(p,\%x y. when(y,\%xa. inl(<x,xa>),?d2(x,y)))}

{\out : (SUM x:A. B(x)) + (SUM x:A. C(x))}

{\out  1. !!x y xa. [| x : A; xa : B(x) |] ==> SUM x:A. C(x) type}

{\out  2. !!x y ya.}

{\out        [| x : A; ya : C(x) |] ==>}

{\out        ?d2(x,y,ya) : (SUM x:A. B(x)) + (SUM x:A. C(x))}

\end{ttbox}

Subgoal~1 is an example of a well-formedness subgoal~\cite{constable86}.

Such subgoals are usually trivial; this one yields to

\ttindex{typechk_tac}, given the current list of premises.

\begin{ttbox}

by (typechk_tac prems);

{\out Level 7}

{\out split(p,\%x y. when(y,\%xa. inl(<x,xa>),?d2(x,y)))}

{\out : (SUM x:A. B(x)) + (SUM x:A. C(x))}

{\out  1. !!x y ya.}

{\out        [| x : A; ya : C(x) |] ==>}

{\out        ?d2(x,y,ya) : (SUM x:A. B(x)) + (SUM x:A. C(x))}

\end{ttbox}

This subgoal is the other case from the $+$-elimination above, and can be

proved similarly.  Quicker is to apply \ttindex{pc_tac}.  The main goal

finally gets a fully instantiated proof object.

\begin{ttbox}

by (pc_tac prems 1);

{\out Level 8}

{\out split(p,\%x y. when(y,\%xa. inl(<x,xa>),\%y. inr(<x,y>)))}

{\out : (SUM x:A. B(x)) + (SUM x:A. C(x))}

{\out No subgoals!}

\end{ttbox}

Calling \ttindex{pc_tac} after the first $\Sigma$-elimination above also

proves this theorem.

\section{Example: deriving a currying functional}

In simply-typed languages such as {\ML}, a currying functional has the type 

\[ (A\times B \to C) \to (A\to (B\to C)). \]

Let us generalize this to the dependent types~$\Sigma$ and~$\Pi$.  

The functional takes a function~$f$ that maps $z:\Sigma(A,B)$

to~$C(z)$; the resulting function maps $x\in A$ and $y\in B(x)$ to

$C(\langle x,y\rangle)$.

Formally, there are three typing premises.  $A$ is a type; $B$ is an

$A$-indexed family of types; $C$ is a family of types indexed by

$\Sigma(A,B)$.  The goal is expressed using \hbox{\tt PROD f} to ensure