------------------------------------------------------------------------
-- Release notes for Agda 2 version 2.2.10
------------------------------------------------------------------------

Important changes since 2.2.8:

Language
--------

* New flag: --without-K.

  This flag makes pattern matching more restricted. If the flag is
  activated, then Agda only accepts certain case-splits. If the type
  of the variable to be split is D pars ixs, where D is a data (or
  record) type, pars stands for the parameters, and ixs the indices,
  then the following requirements must be satisfied:

  * The indices ixs must be applications of constructors to distinct
    variables.

  * These variables must not be free in pars.

  The intended purpose of --without-K is to enable experiments with a
  propositional equality without the K rule. Let us define
  propositional equality as follows:

    data _≡_ {A : Set} : A → A → Set where
      refl : ∀ x → x ≡ x

  Then the obvious implementation of the J rule is accepted:

    J : {A : Set} (P : {x y : A} → x ≡ y → Set) →
        (∀ x → P (refl x)) →
        ∀ {x y} (x≡y : x ≡ y) → P x≡y
    J P p (refl x) = p x

  The same applies to Christine Paulin-Mohring's version of the J rule:

    J′ : {A : Set} {x : A} (P : {y : A} → x ≡ y → Set) →
         P (refl x) →
         ∀ {y} (x≡y : x ≡ y) → P x≡y
    J′ P p (refl x) = p

  On the other hand, the obvious implementation of the K rule is not
  accepted:

    K : {A : Set} (P : {x : A} → x ≡ x → Set) →
        (∀ x → P (refl x)) →
        ∀ {x} (x≡x : x ≡ x) → P x≡x
    K P p (refl x) = p x

  However, we have /not/ proved that activation of --without-K ensures
  that the K rule cannot be proved in some other way.

* Irrelevant declarations.

  Postulates and functions can be marked as irrelevant by prefixing
  the name with a dot when the name is declared. Example:

    postulate
      .irrelevant : {A : Set} → .A → A

  Irrelevant names may only be used in irrelevant positions or in
  definitions of things which have been declared irrelevant.

  The axiom irrelevant above can be used to define a projection from
  an irrelevant record field:

    data Subset (A : Set) (P : A → Set) : Set where
      _#_ : (a : A) → .(P a) → Subset A P

    elem : ∀ {A P} → Subset A P → A
    elem (a # p) = a

    .certificate : ∀ {A P} (x : Subset A P) → P (elem x)
    certificate (a # p) = irrelevant p

  The right-hand side of certificate is relevant, so we cannot define

    certificate (a # p) = p

  (because p is irrelevant). However, certificate is declared to be
  irrelevant, so it can use the axiom irrelevant. Furthermore the
  first argument of the axiom is irrelevant, which means that
  irrelevant p is well-formed.

  As shown above the axiom irrelevant justifies irrelevant
  projections. Previously no projections were generated for irrelevant
  record fields, such as the field certificate in the following
  record type:

    record Subset (A : Set) (P : A → Set) : Set where
      constructor _#_
      field
        elem         : A
        .certificate : P elem

  Now projections are generated automatically for irrelevant fields
  (unless the flag --no-irrelevant-projections is used). Note that
  irrelevant projections are highly experimental.

* Termination checker recognises projections.

  Projections now preserve sizes, both in patterns and expressions.
  Example:

    record Wrap (A : Set) : Set where
      constructor wrap
      field
        unwrap : A

    open Wrap public

    data WNat : Set where
      zero : WNat
      suc  : Wrap WNat → WNat

    id : WNat → WNat
    id zero    = zero
    id (suc w) = suc (wrap (id (unwrap w)))

  In the structural ordering unwrap w ≤ w. This means that

    unwrap w ≤ w < suc w,

  and hence the recursive call to id is accepted.

  Projections also preserve guardedness.

Tools
-----

* Hyperlinks for top-level module names now point to the start of the
  module rather than to the declaration of the module name. This
  applies both to the Emacs mode and to the output of agda --html.

* Most occurrences of record field names are now highlighted as
  "fields". Previously many occurrences were highlighted as
  "functions".

* Emacs mode: It is no longer possible to change the behaviour of the
  TAB key by customising agda2-indentation.

* Epic compiler backend.

  A new compiler backend is being implemented. This backend makes use
  of Edwin Brady's language Epic
  (http://www.cs.st-andrews.ac.uk/~eb/epic.php) and its compiler. The
  backend should handle most Agda code, but is still at an
  experimental stage: more testing is needed, and some things written
  below may not be entirely true.

  The Epic compiler can be invoked from the command line using the
  flag --epic:

    agda --epic --epic-flag=<EPIC-FLAG> --compile-dir=<DIR> <FILE>.agda

  The --epic-flag flag can be given multiple times; each flag is given
  verbatim to the Epic compiler (in the given order). The resulting
  executable is named after the main module and placed in the
  directory specified by the --compile-dir flag (default: the project
  root). Intermediate files are placed in a subdirectory called Epic.

  The backend requires that there is a definition named main. This
  definition should be a value of type IO Unit, but at the moment this
  is not checked (so it is easy to produce a program which segfaults).
  Currently the backend represents actions of type IO A as functions
  from Unit to A, and main is applied to the unit value.

  The Epic compiler compiles via C, not Haskell, so the pragmas
  related to the Haskell FFI (IMPORT, COMPILED_DATA and COMPILED) are
  not used by the Epic backend. Instead there is a new pragma
  COMPILED_EPIC. This pragma is used to give Epic code for postulated
  definitions (Epic code can in turn call C code). The form of the
  pragma is {-# COMPILED_EPIC def code #-}, where def is the name of
  an Agda postulate and code is some Epic code which should include
  the function arguments, return type and function body. As an example
  the IO monad can be defined as follows:

    postulate
      IO     : Set → Set
      return : ∀ {A} → A → IO A
      _>>=_  : ∀ {A B} → IO A → (A → IO B) → IO B

    {-# COMPILED_EPIC return (u : Unit, a : Any) -> Any =
                        ioreturn(a) #-}
    {-# COMPILED_EPIC
          _>>=_ (u1 : Unit, u2 : Unit, x : Any, f : Any) -> Any =
            iobind(x,f) #-}

  Here ioreturn and iobind are Epic functions which are defined in the
  file AgdaPrelude.e which is always included.

  By default the backend will remove so-called forced constructor
  arguments (and case-splitting on forced variables will be
  rewritten). This optimisation can be disabled by using the flag
  --no-forcing.

  All data types which look like unary natural numbers after forced
  constructor arguments have been removed (i.e. types with two
  constructors, one nullary and one with a single recursive argument)
  will be represented as "BigInts". This applies to the standard Fin
  type, for instance.

  The backend supports Agda's primitive functions and the BUILTIN
  pragmas. If the BUILTIN pragmas for unary natural numbers are used,
  then some operations, like addition and multiplication, will use
  more efficient "BigInt" operations.

  If you want to make use of the Epic backend you need to install some
  dependencies, see the README.

* The Emacs mode can compile using either the MAlonzo or the Epic
  backend. The variable agda2-backend controls which backend is used.