diff options
Diffstat (limited to 'HoTT_Theorems.thy')
| -rw-r--r-- | HoTT_Theorems.thy | 67 |
1 files changed, 55 insertions, 12 deletions
diff --git a/HoTT_Theorems.thy b/HoTT_Theorems.thy index 33b0957..d83a08c 100644 --- a/HoTT_Theorems.thy +++ b/HoTT_Theorems.thy @@ -6,23 +6,25 @@ text "A bunch of theorems and other statements for sanity-checking, as well as t Things that *should* be automated: \<bullet> Checking that \<open>A\<close> is a well-formed type, when writing things like \<open>x : A\<close> and \<open>A : U\<close>. -" + \<bullet> Checking that the argument to a (dependent/non-dependent) function matches the type? Also the arguments to a pair?" \<comment> \<open>Turn on trace for unification and the simplifier, for debugging.\<close> -declare[[unify_trace_simp, unify_trace_types, simp_trace]] +declare[[unify_trace_simp, unify_trace_types, simp_trace, simp_trace_depth_limit=2]] section \<open>Functions\<close> +subsection \<open>Typing functions\<close> + text "Declaring \<open>Prod_intro\<close> with the \<open>intro\<close> attribute (in HoTT.thy) enables \<open>standard\<close> to prove the following." -lemma id_function: "A : U \<Longrightarrow> \<^bold>\<lambda>x. x : A\<rightarrow>A" .. +lemma id_function: "A : U \<Longrightarrow> \<^bold>\<lambda>x:A. x : A\<rightarrow>A" .. text "Here is the same result, stated and proved differently. The standard method invoked after the keyword \<open>proof\<close> is applied to the goal \<open>\<^bold>\<lambda>x. x: A\<rightarrow>A\<close>, and so we need to show the prover how to continue, as opposed to the previous lemma." lemma assumes "A : U" - shows "\<^bold>\<lambda>x. x : A\<rightarrow>A" + shows "\<^bold>\<lambda>x:A. x : A\<rightarrow>A" proof show "A : U" using assms . show "\<lambda>x. A : A \<rightarrow> U" using assms .. @@ -31,29 +33,29 @@ qed text "Note that there is no provision for declaring the type of bound variables outside of the scope of a lambda expression. More generally, we cannot write an assumption stating 'Let \<open>x\<close> be a variable of type \<open>A\<close>'." -proposition "\<lbrakk>A : U; A \<equiv> B\<rbrakk> \<Longrightarrow> \<^bold>\<lambda>x. x : B\<rightarrow>A" +proposition "\<lbrakk>A : U; A \<equiv> B\<rbrakk> \<Longrightarrow> \<^bold>\<lambda>x:A. x : B\<rightarrow>A" proof - assume 1: "A : U" and 2: "A \<equiv> B" - from id_function[OF 1] have 3: "\<^bold>\<lambda>x. x : A\<rightarrow>A" . + from id_function[OF 1] have 3: "\<^bold>\<lambda>x:A. x : A\<rightarrow>A" . from 2 have "A\<rightarrow>A \<equiv> B\<rightarrow>A" by simp - with 3 show "\<^bold>\<lambda>x. x : B\<rightarrow>A" .. + with 3 show "\<^bold>\<lambda>x:A. x : B\<rightarrow>A" .. qed text "It is instructive to try to prove \<open>\<lbrakk>A : U; B : U\<rbrakk> \<Longrightarrow> \<^bold>\<lambda>x. \<^bold>\<lambda>y. x : A\<rightarrow>B\<rightarrow>A\<close>. First we prove an intermediate step." -lemma constant_function: "\<lbrakk>A : U; B : U; x : A\<rbrakk> \<Longrightarrow> \<^bold>\<lambda>y. x : B\<rightarrow>A" .. +lemma constant_function: "\<lbrakk>A : U; B : U; x : A\<rbrakk> \<Longrightarrow> \<^bold>\<lambda>y:B. x : B\<rightarrow>A" .. text "And now the actual result:" proposition assumes 1: "A : U" and 2: "B : U" - shows "\<^bold>\<lambda>x. \<^bold>\<lambda>y. x : A\<rightarrow>B\<rightarrow>A" + shows "\<^bold>\<lambda>x:A. \<^bold>\<lambda>y:B. x : A\<rightarrow>B\<rightarrow>A" proof show "A : U" using assms(1) . - show "\<And>x. x : A \<Longrightarrow> \<^bold>\<lambda>y. x : B \<rightarrow> A" using assms by (rule constant_function) + show "\<And>x. x : A \<Longrightarrow> \<^bold>\<lambda>y:B. x : B \<rightarrow> A" using assms by (rule constant_function) from assms have "B \<rightarrow> A : U" by (rule Prod_formation) then show "\<lambda>x. B \<rightarrow> A: A \<rightarrow> U" using assms(1) by (rule constant_type_family) @@ -61,13 +63,54 @@ qed text "Maybe a nicer way to write it:" -proposition "\<lbrakk>A : U; B: U\<rbrakk> \<Longrightarrow> \<^bold>\<lambda>x. \<^bold>\<lambda>y. x : A\<rightarrow>B\<rightarrow>A" +proposition alternating_function: "\<lbrakk>A : U; B: U\<rbrakk> \<Longrightarrow> \<^bold>\<lambda>x:A. \<^bold>\<lambda>y:B. x : A\<rightarrow>B\<rightarrow>A" proof fix x - show "\<lbrakk>A : U; B : U; x : A\<rbrakk> \<Longrightarrow> \<^bold>\<lambda>y. x : B \<rightarrow> A" by (rule constant_function) + show "\<lbrakk>A : U; B : U; x : A\<rbrakk> \<Longrightarrow> \<^bold>\<lambda>y:B. x : B \<rightarrow> A" by (rule constant_function) show "\<lbrakk>A : U; B : U\<rbrakk> \<Longrightarrow> B\<rightarrow>A : U" by (rule Prod_formation) qed +subsection \<open>Function application\<close> + +lemma "\<lbrakk>A : U; a : A\<rbrakk> \<Longrightarrow> (\<^bold>\<lambda>x:A. x)`a \<equiv> a" by simp + +lemma + assumes + "A:U" and + "B:U" and + "a:A" and + "b:B" + shows "(\<^bold>\<lambda>x:A. \<^bold>\<lambda>y:B. x)`a`b \<equiv> a" +proof - + have "(\<^bold>\<lambda>x:A. \<^bold>\<lambda>y:B. x)`a \<equiv> \<^bold>\<lambda>y:B. a" + proof (rule Prod_comp[of A "\<lambda>_. B\<rightarrow>A"]) + have "B \<rightarrow> A : U" using constant_type_family[OF assms(1) assms(2)] assms(2) by (rule Prod_formation) + then show "\<lambda>x. B \<rightarrow> A: A \<rightarrow> U" using assms(1) by (rule constant_type_family[of "B\<rightarrow>A"]) + + show "\<And>x. x : A \<Longrightarrow> \<^bold>\<lambda>y:B. x : B \<rightarrow> A" using assms(2) assms(1) .. + show "A:U" using assms(1) . + show "a:A" using assms(3) . + qed (* Why do I need to do the above for the last two goals? Can't Isabelle do it automatically? *) + + then have "(\<^bold>\<lambda>x:A. \<^bold>\<lambda>y:B. x)`a`b \<equiv> (\<^bold>\<lambda>y:B. a)`b" by simp + + also have "(\<^bold>\<lambda>y:B. a)`b \<equiv> a" + proof (rule Prod_comp[of B "\<lambda>_. A"]) + show "\<lambda>y. A: B \<rightarrow> U" using assms(1) assms(2) by (rule constant_type_family) + show "\<And>y. y : B \<Longrightarrow> a : A" using assms(3) . + show "B:U" using assms(2) . + show "b:B" using assms(4) . + qed + + finally show "(\<^bold>\<lambda>x:A. \<^bold>\<lambda>y:B. x)`a`b \<equiv> a" . +qed + +text "Polymorphic identity function." + +consts Ui::Term +definition Id where "Id \<equiv> \<^bold>\<lambda>A:Ui. \<^bold>\<lambda>x:A. x" +(* Have to think about universes... *) + section \<open>Nats\<close> text "Here's a dumb proof that 2 is a natural number." |