Theory Computations

(*  Title:      HOL/ex/Computations.thy
    Author:     Florian Haftmann, TU Muenchen
*)

section ‹Simple example for computations generated by the code generator›

theory Computations
  imports Main
begin

fun even :: "nat ⇒ bool"
  where "even 0 ⟷ True"
      | "even (Suc 0) ⟷ False"
      | "even (Suc (Suc n)) ⟷ even n"
  
fun fib :: "nat ⇒ nat"
  where "fib 0 = 0"
      | "fib (Suc 0) = Suc 0"
      | "fib (Suc (Suc n)) = fib (Suc n) + fib n"

declare [[ML_source_trace]]

ML ‹
local 

fun int_of_nat @{code "0 :: nat"} = 0
  | int_of_nat (@{code Suc} n) = int_of_nat n + 1;

in

val comp_nat = @{computation nat
  terms: "plus :: nat ⇒_" "times :: nat ⇒ _" fib
  datatypes: nat}
  (fn post => post o HOLogic.mk_nat o int_of_nat o the);

val comp_numeral = @{computation nat
  terms: "0 :: nat" "1 :: nat" "2 :: nat" "3 :: nat"}
  (fn post => post o HOLogic.mk_nat o int_of_nat o the);

val comp_bool = @{computation bool
  terms: HOL.conj HOL.disj HOL.implies
    HOL.iff even "less_eq :: nat ⇒ _" "less :: nat ⇒ _" "HOL.eq :: nat ⇒ _"
  datatypes: bool}
  (K the);

val comp_check = @{computation_check terms: Trueprop};

val comp_dummy = @{computation "(nat × unit) option"
  datatypes: "(nat × unit) option"}

end
›

declare [[ML_source_trace = false]]
  
ML_val ‹
  comp_nat @{context} @{term "fib (Suc (Suc (Suc 0)) * Suc (Suc (Suc 0))) + Suc 0"}
  |> Syntax.string_of_term @{context}
  |> writeln
›
  
ML_val ‹
  comp_bool @{context} @{term "fib (Suc (Suc (Suc 0)) * Suc (Suc (Suc 0))) + Suc 0 < fib (Suc (Suc 0))"}
›

ML_val ‹
  comp_check @{context} @{cprop "fib (Suc (Suc (Suc 0)) * Suc (Suc (Suc 0))) + Suc 0 > fib (Suc (Suc 0))"}
›
  
ML_val ‹
  comp_numeral @{context} @{term "Suc 42 + 7"}
  |> Syntax.string_of_term @{context}
  |> writeln
›

end