# Chapter 1 Functional Programming in Coq

## Data and Functions

Coq’s set of built-in features is extremely small, we could define new data-types from scratch ourselves.

Every expression in Coq has a type, describing what sort of thing it computes.

Coq allows us to create data-types by explicitly enumerating a finite set of elements, called constructors. Constructor expressions are formed by applying to a constructor zero or more other constructor expressions.

To test the correctness of a function, we check that it works on some examples. We could (1) use Compute to evaluate an expression, (2) use Example with the expected result given, then prove by tactics, (3) extract the function to another language like Haskell.

Use Fixpoint when having to use the data-type definition itself in its constructors.

Use Notation to introduce our own notations; we could make several annotations to control how these notations are treated by Coq’s parser, for example associativity and scope.

## Proof by Simplification

Although reflexivity does more simplifications than simpl, we sometimes need simpl to read and understand the new goal it creates, so as to not leaving the goal in a messy state.

Use intros tactic to introduce a quantifier in the goals to context of current assumptions. If the quantifier is $\forall n:\mathbb{N}$, intros n is like saying “Suppose $n$ is some number…”

## Proof by Rewriting

The intros tactic also allows us to introduce the hypothesis to current assumptions. If the hypothesis is an equality, we could replace one side of the equality with another.

When writing long proofs, its desirable to use Admitted to temporarily accept a subsidiary lemma; then we can go back and fill in the proofs.

## Proof by Case Analysis

The destruct tactic can be used with any inductively defined data-type; this tactic would separate the original goal into several subgoals, amounting to the number of constructors of the data-type. Using this tactic, we could annotate variable names using intro pattern, separating lists of names by | and give name to subgoal equations using the eqn: qualifier.

The bullets mark the parts of the proof correspond to subgoals, preventing proofs for different subgoals from getting mixed up.