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{{DISPLAYTITLE:Propositions As Types}}
 
{{DISPLAYTITLE:Propositions As Types}}
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'''NB.''' In this discussion, combinators are being applied on the right of their arguments.  The resulting formulas will look backwards to people who are accustomed to applying combinators on the left.
    
==Identity, or the Identifier==
 
==Identity, or the Identifier==
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==Bibliography==
 
==Bibliography==
   −
Here are a three references on combinatory logic and lambda calculus, given in order of difficulty from introductory to advanced, that are especially pertinent to the use of combinators in computer science:
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Here are three references on combinatory logic and lambda calculus, given in order of difficulty from introductory to advanced, that are especially pertinent to the use of combinators in computer science:
    
# Smullyan, R. (1985), ''To Mock a Mockingbird, And Other Logic Puzzles, Including an Amazing Adventure in Combinatory Logic'', Alfred A. Knopf, New York, NY.
 
# Smullyan, R. (1985), ''To Mock a Mockingbird, And Other Logic Puzzles, Including an Amazing Adventure in Combinatory Logic'', Alfred A. Knopf, New York, NY.
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::<p><math>\begin{array}{c}
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::<p><math>\dfrac{~ f : A \to B \quad g : B \to C ~}{gf : A \to C}</math></p>
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\underline{~ f : A \to B ~~~~~ g : B \to C ~}
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\\
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gf : A \to C
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\end{array}</math></p>
   
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|-
 
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<p>(Lambek & Scott, 5).
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<p>(Lambek & Scott, 5).</p>
 
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|}
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<p><math>\text{R1a.} \quad A \xrightarrow{~1_A~} A,</math></p>
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<p><math>\text{R1a.} \quad A ~\xrightarrow{~1_A~}~ A,</math></p>
 
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|-
 
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<p><math>\text{R1b.} \quad \dfrac{A \xrightarrow{~f~} B \quad B \xrightarrow{~g~} C}{A \xrightarrow{~gf~} C}.</math></p>
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<p><math>\text{R1b.} \quad \dfrac{~ A ~\xrightarrow{~f~}~ B \quad B ~\xrightarrow{~g~}~ C ~}{A ~\xrightarrow{~gf~}~ C}.</math></p>
 
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===Conjunction Calculus===
 
===Conjunction Calculus===
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<pre>
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{| align="center" cellpadding="8" width="90%" <!--QUOTE-->
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| A 'conjunction calculus' is a deductive system dealing with truth and
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| conjunction.  Thus we assume that there is given a formula 'T' (= true)
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| and a binary operation '&' (= and) for forming the conjunction A & B of
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| two given formulas A and B.  Moreover, we specify the following additional
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| arrows and rules of inference:
   
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|
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|          O_A
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<p>A ''conjunction calculus'' is a deductive system dealing with truth and conjunction. Thus we assume that there is given a formula <math>\operatorname{T}</math> (&nbsp;=&nbsp;true) and a binary operation <math>\land</math> (&nbsp;=&nbsp;and) for forming the conjunction <math>A \land B</math> of two given formulas <math>A\!</math> and <math>B.\!</math> Moreover, we specify the following additional arrows and rules of inference:</p>
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| R2.   A -----> T,
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|-
 
|
 
|
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|              p1_A,B
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<p><math>\begin{array}{ll}
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| R3a.   A & B --------> A,
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\text{R2.}  & A ~\xrightarrow{~\bigcirc_A~}~ \operatorname{T};
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\\[8pt]
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\text{R3a.} & A \land B ~\xrightarrow{~\pi_{A, B}~}~ A,
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\\[8pt]
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\text{R3b.} & A \land B ~\xrightarrow{~\pi'_{A, B}~}~ B,
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\\[8pt]
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\text{R3c.} & \dfrac{~ C ~\xrightarrow{~f~}~ A \quad C ~\xrightarrow{~g~}~ B ~}{C ~\xrightarrow{~\langle f, g \rangle~}~ A \land B}.
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\end{array}</math></p>
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|-
 
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|
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|              p2_A,B
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<p>(Lambek & Scott, 47&ndash;48).</p>
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| R3b.  A & B --------> B,
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|}
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|
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|          f          g
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|        C ---> A    C ---> B
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| R3c.  ----------------------.
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|          <f, g>
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|       C --------> A & B
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</pre>
      
===Positive Intuitionistic Propositional Calculus===
 
===Positive Intuitionistic Propositional Calculus===
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<pre>
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{| align="center" cellpadding="8" width="90%" <!--QUOTE-->
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| A 'positive intuitionistic propositional calculus' is a conjunction calculus
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| with an additional binary operation '<=' (= if).  Thus, if A and B are formulas,
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| so are T, A & B, and A <= B.  (Yes, most people write B => A instead.)  We also
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| specify the following new arrow and rule of inference:
   
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|
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|                    !e!_A,B
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<p>A ''positive intuitionistic propositional calculus'' is a conjunction calculus with an additional binary operation <math>\Leftarrow</math> (&nbsp;=&nbsp;if).  Thus, if <math>A\!</math> and <math>B\!</math> are formulas, so are <math>\operatorname{T},</math> <math>A \land B,</math> and <math>A \Leftarrow B.</math> (Yes, most people write <math>B \Rightarrow A</math> instead.) We also specify the following new arrow and rule of inference.</p>
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| R4a.  (A <= B) & B ---------> A,
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|-
 
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|
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|              h
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<p><math>\begin{array}{ll}
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|        C & B ---> A
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\text{R4a.} & (A \Leftarrow B) \land B ~\xrightarrow{~\varepsilon_{A, B}~}~ A,
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| R4b. ----------------.
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\\[8pt]
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|          h*
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\text{R4b.} & \dfrac{~ C \land B ~\xrightarrow{~h~}~ A ~}{~ C ~\xrightarrow{~h^*~}~ A \Leftarrow B ~}.
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|       C ----> A <= B
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\end{array}</math></p>
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|-
 
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|
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</pre>
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<p>(Lambek & Scott, 48&ndash;49).</p>
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|}
    
===Intuitionistic Propositional Calculus===
 
===Intuitionistic Propositional Calculus===
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<pre>
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{| align="center" cellpadding="8" width="90%" <!--QUOTE-->
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| An 'intuitionistic propositional calculus' is more than a
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| positive one;  it requires also falsehood and disjunction,
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| that is, a formula 'F' (= false) and an operation 'v' (= or)
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| on formulas, together with the following additional arrows:
   
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|
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|          []_A
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<p>An ''intuitionistic propositional calculus'' is more than a positive one;  it requires also falsehood and disjunction, that is, a formula <math>\bot</math> (&nbsp;=&nbsp;false) and an operation <math>\lor</math> (&nbsp;=&nbsp;or) on formulas, together with the following additional arrows:</p>
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| R5.    F ------> A,
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|-
 
|
 
|
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|          k1_A,B
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<p><math>\begin{array}{ll}
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| R6a.   A --------> A v B,
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\text{R5.}  & \bot ~\xrightarrow{~\Box_A~}~ A;
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\\[8pt]
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\text{R6a.} & A ~\xrightarrow{~\kappa_{A, B}~}~ A \lor B,
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\\[8pt]
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\text{R6b.} & B ~\xrightarrow{~\kappa'_{A, B}~}~ A \lor B,
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\\[8pt]
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\text{R6c.} & (C \Leftarrow A) \land (C \Leftarrow B) ~\xrightarrow{~\zeta^C_{A, B}~}~ C \Leftarrow (A \lor B).
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\end{array}</math></p>
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|-
 
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|
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|          k2_A,B
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<p>(Lambek & Scott, 49&ndash;50).</p>
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| R6b.  B --------> A v B,
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|}
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|
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|                            !z!^C_A,B
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| R6c.  (C <= A) & (C <= B) -----------> C <= (A v B).
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</pre>
      
===Classical Propositional Calculus===
 
===Classical Propositional Calculus===
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<pre>
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{| align="center" cellpadding="8" width="90%" <!--QUOTE-->
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| If we want 'classical' propositional logic, we must also require:
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|
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<p>If we want ''classical'' propositional logic, we must also require:
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|-
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|
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<p><math>\begin{array}{ll}
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\text{R7.} & (\bot \Leftarrow (\bot \Leftarrow A)) \to A.
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\end{array}</math></p>
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|-
 
|
 
|
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| R7.  F <= (F <= A) -> A.
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<p>(Lambek & Scott, 50).</p>
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</pre>
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|}
    
===Category 2===
 
===Category 2===
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<pre>
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{| align="center" cellpadding="8" width="90%" <!--QUOTE-->
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| A 'category' is a deductive system in which
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| the following equations hold between proofs:
   
|
 
|
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| E1.  f 1_A  =  f,
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<p>A ''category'' is a deductive system in which the following equations hold between proofs:</p>
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|-
 
|
 
|
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|      1_B f = f,
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<p><math>\begin{array}{ll}
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\text{E1.} & f 1_A = f, \qquad 1_B f = f, \qquad (hg)f = h(gf),
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\\[8pt]
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& \text{for all}~ f : A \to B, \quad g : B \to C, \quad h : C \to D.
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\end{array}</math></p>
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|-
 
|
 
|
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|      (hg)f  =  h(gf),
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<p>(Lambek & Scott, 52).</p>
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|}
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| for all f : A -> B, g : B -> C, h : C -> D.
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</pre>
      
===Cartesian Category===
 
===Cartesian Category===
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<pre>
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{| align="center" cellpadding="8" width="90%" <!--QUOTE-->
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| A 'cartesian category' is both a category
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| and a conjunction calculus satisfying the
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| additional equations:
   
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|
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| E2.  f  =  O_A,  for all f : A -> T.
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<p>A ''cartesian category'' is both a category and a conjunction calculus satisfying the additional equations:</p>
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|-
 
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|
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| E3a. p1_A,B <f, g>  = f,
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<p><math>\begin{array}{ll}
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\text{E2.}  & f = \bigcirc_A, \quad \text{for all}~ f : A \to \operatorname{T};
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\\[8pt]
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\text{E3a.} & \pi^{}_{A,B} \langle f, g \rangle = f,
 +
\\[8pt]
 +
\text{E3b.} & \pi^\prime_{A,B} \langle f, g \rangle = g,
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\\[8pt]
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\text{E3c.} & \langle \pi^{}_{A,B} h, \pi^\prime_{A,B} h \rangle = h,
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\\[8pt]
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& \text{for all}~ f : C \to A, \quad g : C \to B, \quad h : C \to A \land B.
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\end{array}</math></p>
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|-
 
|
 
|
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| E3b.  p2_A,B <f, g> =  g,
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<p>(Lambek & Scott, 52).</p>
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|
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|}
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| E3c. <p1_A,B h, p2_A,B h> =  h,
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|
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| for all f : C -> A, g : C -> B, h : C -> A & B.
  −
</pre>
      
===Cartesian Closed Category===
 
===Cartesian Closed Category===
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<pre>
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{| align="center" cellpadding="8" width="90%" <!--QUOTE-->
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| A 'cartesian closed category' is a cartesian category $A$ with
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| additional structure R4 satisfying the additional equations:
   
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| E4a.  !e!_A,B <h* p1_C,B, p2_C,B>   =  h,
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<p>A ''cartesian closed category'' is a cartesian category <math>\mathcal{A}</math> with additional structure <math>\text{R4}\!</math> satisfying the additional equations:</p>
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|-
 
|
 
|
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| E4b. (!e!_A,B <k p1_C,B, p2_C,B>)* = k,
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<p><math>\begin{array}{ll}
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\text{E4a.} & \varepsilon^{}_{A,B} \langle h^* \pi^{}_{C,B}, \pi^\prime_{C,B} \rangle = h,
 +
\\[8pt]
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\text{E4b.} & (\varepsilon^{}_{A,B} \langle k \pi^{}_{C,B}, \pi^\prime_{C,B} \rangle)^* = k,
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\\[8pt]
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& \text{for all}~ h : C \land B \to A \quad \text{and} \quad k : C \to (A \Leftarrow B).
 +
\end{array}</math></p>
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|-
 
|
 
|
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| for all h : C & B -> A,  k : C -> (A <= B).
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<p>Thus, a cartesian closed category is a positive intuitionistic propositional calculus satisfying the equations <math>\text{E1}\!</math> to <math>\text{E4}.\!</math>  This illustrates the general principle that one may obtain interesting categories from deductive systems by imposing an appropriate equivalence relation on proofs.</p>
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|
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| Thus, a cartesian closed category is
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<p>(Lambek & Scott, 53).</p>
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| a positive intuitionistic propositional
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|}
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| calculus satisfying the equations E1 to E4.
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| This illustrates the general principle that
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| one may obtain interesting categories from
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| deductive systems by imposing an appropriate
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| equivalence relation on proofs.
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</pre>
      
==Document History==
 
==Document History==
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# http://stderr.org/pipermail/inquiry/2005-July/002895.html
 
# http://stderr.org/pipermail/inquiry/2005-July/002895.html
 
# http://stderr.org/pipermail/inquiry/2005-July/002896.html
 
# http://stderr.org/pipermail/inquiry/2005-July/002896.html
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[[Category:Combinator Calculus]]
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[[Category:Combinatory Logic]]
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[[Category:Computer Science]]
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[[Category:Graph Theory]]
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[[Category:Lambda Calculus]]
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[[Category:Logic]]
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[[Category:Logical Graphs]]
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[[Category:Mathematics]]
 +
[[Category:Programming Languages]]
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[[Category:Type Theory]]
Jon Awbrey
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