Changelog for Agda-2.6.2

Release notes for Agda version 2.6.2

Highlights

Several improvements and bug-fixes related to Run-time Irrelevance.
Several improvements and bug-fixes related to the JavaScript Backend.
Added experimental support for Guarded Cubical Agda.
The Primitive Sorts of Agda (Set and Prop) are no longer keywords and can be renamed when importing Agda.Primitive.
Added native support for the Inspect Idiom.
Added support for making System Calls from the reflection API.

Installation and infrastructure

Added support for GHC 8.10.5 and 9.0.1.
Some expensive optimisations are now off by default (see #4521).

These optimisations can in some cases make Agda substantially faster, but they can also make the compilation of the Agda program take more time and space.

The optimisations can be turned on manually (Cabal: -foptimise-heavily, Stack: --flag Agda:optimise-heavily). They are turned on (by default) when Agda is installed using make install.

If the optimisations are turned on it might make sense to limit GHC's memory usage (using something like --ghc-options="+RTS -M6G -RTS").

Pragmas and options

New option --auto-inline turns on automatic compile-time inlining of simple functions. This was previously enabled by default.

Note that the absence of automatic inlining can make typechecking substantially slower.

The new default has repercussions on termination checking, for instance (see #4702). The following formulation of plus termination checks with --auto-inline but not without:
```
open import Agda.Builtin.Nat

case_of_ : {A B : Set} → A → (A → B) → B
case x of f = f x

plus : Nat → Nat → Nat
plus m n = case m of λ
   { zero    → n
   ; (suc m) → suc (plus m n)
   }
```
In this particular case, we can work around the limitation of the termination checker with pragma {-# INLINE case_of_ #-}.
New options --qualified-instances (default) and --no-qualified-instances. When --no-qualified-instances is enabled, Agda will only consider candidates for instance search that are in scope under an unqualified name (see #4522).
New option --call-by-name turns off call-by-need evaluation at type checking time.
New option --highlight-occurrences (off by default) enables the HTML backend to include a JavaScript file that highlights all occurrences of the mouse-hovered symbol (see #4535).
New option --no-import-sorts disables the implicit open import Agda.Primitive using (Set; Prop) at the top of each file (see below).
New option --local-confluence-check to restore the old behaviour of the --confluence-check flag (see below for the new behaviour).
New primitive primStringFromListInjective internalising the fact that primStringFromList is an injective function. It is bound in Agda.Builtin.String.Properties.
New option --allow-exec enables the use of system calls during type checking using the AGDATCMEXECTC builtin.
New option --show-identity-substitutions shows all arguments of metavariables when pretty-printing a term, even if they amount to just applying all the variables in the context.
The option --rewriting is now considered infective: if a module has --rewriting enabled, then all modules importing it must also have --rewriting enabled.
New option --no-double-check (default), opposite of the existing --double-check.
Due to several known soundness issues with sized types (see #1201, #1946, #2820, #3026), the --sized-types flag can no longer be used while --safe is active.
New option --guarded turns on the Guarded Cubical extension of Agda.

See Guarded Cubical in the documentation for more.
The flags --guardedness and --sized-types are no longer enabled by default.

Command-line interaction

In the previous release, Agda exited with either status 0 when the program type checks successfully, or status 1 when encountering any kind of error. Now Agda exits with status 42 for type errors, 71 for errors in the commandline arguments, and 154 for impossible errors. Exit status 1 may be returned under other circumstances; for instance, an incomplete pattern matching, or an error generated by the Haskell runtime. See PR #4540.

Lexical syntax

Layout handling has been improved so that block starters can be stacked on the same line #1145.

If several layout blocks are started by layout keywords without line break in between (where line breaks inside block comments do not count), then those blocks indented more than the last block go passive, meaning they cannot be further extended by new statements.
```
  private module M where postulate
            A : Set                 -- module-block goes passive
            B : Set                 -- postulate-block can still be extended
          module N where            -- private-block can still be extended
```
Previously, this was a parse error.

Language

Inductive records without η-equality no longer support both matching on the record constructor and construction of record elements by copattern matching. It has been discovered that the combination of both leads to loss of subject reduction, i.e., reduction does not preserve typing. See issue #4560.

η-equality for a record can be turned off manually with directive no-eta-equality or command-line option --no-eta-equality, but it is also automatically turned off for some recursive records. For records without η, matching on the record constructor is now off by default and construction by copattern matching is on. If you want the converse, you can add the new record directive pattern.

Example with record pattern:
```
record N : Set where
  inductive
  no-eta-equality
  pattern
  field out : Maybe N

pred : N → Maybe N
pred record{ out = m } = m
```
Example with record constructor and use of ; instead of newline:
```
record N : Set where
  inductive; no-eta-equality
  pattern; constructor inn
  field out : Maybe N

pred : N → Maybe N
pred (inn m) = m
```
Set and Prop are no longer keywords but are now primitives defined in the module Agda.Primitive. They can be renamed when importing this module, for example:
```
open import Agda.Primitive renaming (Set to Type)

test : Type₁
test = Type
```
To preserve backwards compatibility, each top-level Agda module now starts with an implicit statement:
```
open import Agda.Primitive using (Set; Prop)
```
This implicit import can be disabled with the --no-import-sorts flag.
Agda now has support for sorts Setωᵢ (alternative syntax: Setωi) for natural numbers i, where Setω₀ = Setω. These sorts form a second hierarchy Setωᵢ : Setωᵢ₊₁ similar to the standard hierarchy of Setᵢ, but do not support universe polymorphism. It should not be necessary to refer to these sorts during normal usage of Agda, but they might be useful for defining reflection-based macros (see #2119 and #4585).
Changed the internal representation of literal strings: instead of using a linked list of characters (String), we are now using Data.Text. This should be a transparent change from the user's point of view: the backend was already packing these strings as text.

Used this opportunity to introduce a primStringUncons primitive in Agda.Builtin.String (and to correspondingly add the Agda.Builtin.Maybe it needs).
The option --confluence-check for rewrite rules has been given a new implementation that checks global confluence instead of local confluence. Concretely, it does so by enforcing two properties:
1. For any two left-hand sides of the rewrite rules that overlap (either at the root position or at a subterm), the most general unifier of the two left-hand sides is again a left-hand side of a rewrite rule. For example, if there are two rules suc m + n = suc (m + n) and m + suc n = suc (m + n), then there should also be a rule suc m + suc n = suc (suc (m + n)).
2. Each rewrite rule should satisfy the triangle property: For any rewrite rule u = w and any single-step parallel unfolding u => v, we should have another single-step parallel unfolding v => w.
The previous behaviour of the confluence checker that only ensures local confluence can be restored by using the --local-confluence-check flag.
Binary integer literals with prefix 0b (for instance, 0b11001001) are now supported.
Overloaded literals now require the conversion function (fromNat, fromNeg, or fromString) to be in scope unqualified to take effect.

Previously, it was enough for the function to be in scope at all, which meant you couldn't import the corresponding builtin module without having overloaded literals turned on.

Added interleaved mutual blocks where users can forward-declare function, record, and data types and interleave their definitions. These blocks are elaborated to more traditional mutual blocks by:

leaving the signatures where they are
grouping the clauses for a function together with the first of them
grouping the constructors for a datatype together with the first of them

Example: two interleaved function definitions


interleaved mutual

  -- Declarations:
  even : Nat → Bool
  odd  : Nat → Bool

  -- zero is even, not odd
  even zero = true
  odd  zero = false

  -- suc case: switch evenness on the predecessor
  even (suc n) = odd n
  odd  (suc n) = even n

Other example: the definition of universe of types closed under the natural numbers and pairing:


interleaved mutual

  -- Declaration of a product record, a universe of codes, and a decoding function
  record _×_ (A B : Set) : Set
  data U : Set
  El : U → Set

  -- We have a code for the type of natural numbers in our universe
  constructor `Nat : U
  El `Nat = Nat

  -- Btw we know how to pair values in a record
  record _×_ A B where
    constructor _,_
    inductive
    field fst : A; snd : B

  -- And we have a code for pairs in our universe
  constructor _`×_ : (A B : U) → U
  El (A `× B) = El A × El B

Erased constructors (see #4638).

Constructors can be marked as erased. Example:

  {-# OPTIONS --cubical --safe #-}

  open import Agda.Builtin.Cubical.Path
  open import Agda.Primitive

  private
    variable
      a   : Level
      A B : Set a

  Is-proposition : Set a → Set a
  Is-proposition A = (x y : A) → x ≡ y

  data ∥_∥ (A : Set a) : Set a where
    ∣_∣        : A → ∥ A ∥
    @0 trivial : Is-proposition ∥ A ∥

  rec : @0 Is-proposition B → (A → B) → ∥ A ∥ → B
  rec p f ∣ x ∣           = f x
  rec p f (trivial x y i) = p (rec p f x) (rec p f y) i

In the code above the constructor trivial is only available at compile-time, whereas ∣_∣ is also available at run-time. Erased names can be used in bodies of clauses that match on trivial, if the match is done in a non-erased position, like in the final clause of rec. (Note that Cubical Agda programs still cannot be compiled.)

Erased pattern-matching lambdas (see #4525).

Regular pattern-matching lambdas are treated as non-erased function definitions. One can make a pattern-matching lambda erased by writing @0 or @erased after the lambda:
```
@0 _ : @0 Set → Set
_ = λ @0 { A → A }

@0 _ : @0 Set → Set
_ = λ @erased where
  A → A
```
The reflection machinery currently does not support erased pattern-matching lambdas (they are quoted as regular pattern-matching lambdas).
New (?) rule for modalities of generalised variables (see #5058).

The new rule is that generalisable variables get the modality that they are declared with, whereas other variables always get the default modality. (It is unclear what the old rule was, perhaps nothing was changed.)
Private abstract type signatures can no longer see through abstract (see #418).

This means that abstract definitions no longer evaluate in any type signatures in the same module. Previously they evaluated in type signatures of definitions that were both private and abstract.

It also means that metavariables in type signatures have to be solved locally, and cannot make use of information in the definition body, and that constructors of abstract datatypes are not in scope in type signatures.
Type inference is disabled for abstract definitions (see #418).

This means that abstract definitions (inluding functions defined in where blocks of abstract definitions) need complete type signatures.
One can now declare syntax with two name parts without any hole in between, and syntax without any holes.

Examples:
```
syntax Σ A (λ x → B) = [ x ∶ A ] × B
syntax []            = [ ]
```
Internalised the inspect idiom that allows users to abstract over an expression in a with clause while, at the same time, remembering the origin of the abstracted pattern via an equation.

In the following example, abstracting over and then matching on the result of p x allows the first call to filter p (x ∷ xs) to reduce.

In case the element x is kept, the second call to filter on the LHS then performs the same p x test. Because we have retained the proof that p x ≡ true in eq, we are able to rewrite by this equality and get it to reduce too.

This leads to just enough computation that we can finish the proof with an appeal to congruence and the induction hypothesis.
```
filter-filter : ∀ p xs → filter p (filter p xs) ≡ filter p xs
filter-filter p []       = refl
filter-filter p (x ∷ xs) with p x in eq
... | false = filter-filter p xs -- easy
... | true -- second filter stuck on `p x`: rewrite by `eq`!
  rewrite eq = cong (x ∷_) (filter-filter p xs)
```
As a consequence of the above extensions to with, lambdas and lets now need parentheses when appearing in a with. For instance,
```
with-on-fun : Nat → Nat
with-on-fun n with (λ m → m + n)  -- parentheses required!
... | f = f n
```

It is now possible to add hiding and relevance annotations to with expressions. For example:

module _ (A B : Set) (recompute : .B → .{{A}} → B) where

  _$_ : .(A → B) → .A → B
  f $ x with .{f} | .(f x) | .{{x}}
  ... | y = recompute y

Builtins

Primitive operations for floating-point numbers changed. The equalities now follow IEEE 754 equality, after unifying all NaNs. Primitive inequality was added:

primFloatEquality   : Float -> Float -> Bool -- from primFloatNumericEquality
primFloatLess       : Float -> Float -> Bool -- from primFloatNumericLess
primFloatInequality : Float -> Float -> Bool -- new

The “numeric” relations are now deprecated.

There are several new predicates on floating-point numbers:

primFloatIsInfinite     : Float -> Bool -- new
primFloatIsNaN          : Float -> Bool -- new
primFloatIsSafeInteger  : Float -> Bool -- new

The primFloatIsSafeInteger function determines whether the value is a number that is a safe integer, i.e., is within the range where the arithmetic operations do not lose precision.

The operations for conversion to integers (primRound, primFloor, and primCeiling) were renamed for consistency, and return a value of type Maybe Int, returning nothing for NaN and the infinities:

primFloatRound   : Float → Maybe Int -- from primRound
primFloatFloor   : Float → Maybe Int -- from primFloor
primFloatCeiling : Float → Maybe Int -- from primCeiling

There are several new conversions:

primIntToFloat    : Int -> Float               -- new
primFloatToRatio  : Float -> (Int × Nat)       -- new
primRatioToFloat  : Int -> Nat -> Float        -- new
primFloatDecode   : Float -> Maybe (Int × Int) -- new
primFloatEncode   : Int -> Int -> Maybe Float  -- new

The primFloatDecode function decodes a floating-point number f to a mantissa and exponent, such that f = mantissa * 2 ^ exponent, normalised such that the mantissa is the smallest possible number. The primFloatEncode function encodes a pair of a mantissa and exponent to a floating-point number.

There are several new operations:

primFloatPow        : Float -> Float -> Float -- new
primFloatATan2      : Float -> Float -> Float -- from primATan2
primFloatSinh       : Float -> Float          -- new
primFloatCosh       : Float -> Float          -- new
primFloatTanh       : Float -> Float          -- new
primFloatASinh      : Float -> Float          -- new
primFloatACosh      : Float -> Float          -- new
primFloatATanh      : Float -> Float          -- new

Furthermore, the following operations were renamed for consistency:

primFloatExp        : Float -> Float          -- from primExp
primFloatSin        : Float -> Float          -- from primSin
primFloatLog        : Float -> Float          -- from primLog
primFloatCos        : Float -> Float          -- from primCos
primFloatTan        : Float -> Float          -- from primTan
primFloatASin       : Float -> Float          -- from primASin
primFloatACos       : Float -> Float          -- from primACos
primFloatATan       : Float -> Float          -- from primATan

All of these operations are implemented on the JavaScript backend.

primNatToChar maps surrogate code points to the replacement character 'U+FFFD and surrogate code points are disallowed in character literals

Surrogate code points are characters in the range U+D800 to U+DFFF and are reserved for use by UTF-16.

The reason for this change is that strings are represented (at type-checking time and in the GHC backend) by Data.Text byte strings, which cannot represent surrogate code points and replaces them by U+FFFD. By doing the same for characters we can have primStringFromList be injective (witnessed by Agda.Builtin.String.Properties.primStringFromListInjective).

Reflection

New operation in TC monad, similar to quoteTC but operating on types in Setω
```
quoteωTC : ∀ {A : Setω} → A → TC Term
```
typeError and debugPrint no longer inserts spaces around termErr and nameErr parts. They also do a better job of respecting line breaks in strErr parts.

The reflection machinery now supports quantities in Arg (see #5317). The ArgInfo type has changed, and there are new types Modality and Quantity:

data Quantity : Set where
  quantity-0 quantity-ω : Quantity

{-# BUILTIN QUANTITY   Quantity   #-}
{-# BUILTIN QUANTITY-0 quantity-0 #-}
{-# BUILTIN QUANTITY-ω quantity-ω #-}

data Modality : Set where
  modality : (r : Relevance) (q : Quantity) → Modality

{-# BUILTIN MODALITY             Modality #-}
{-# BUILTIN MODALITY-CONSTRUCTOR modality #-}

data ArgInfo : Set where
  arg-info : (v : Visibility) (m : Modality) → ArgInfo

The representation of reflected patterns and clauses has changed. Each clause now includes a telescope with the names and types of the pattern variables.
```
data Clause where
  clause        : (tel : List (Σ String λ _ → Arg Type)) (ps : List (Arg Pattern)) (t : Term) → Clause
  absurd-clause : (tel : List (Σ String λ _ → Arg Type)) (ps : List (Arg Pattern)) → Clause
```
These telescopes provide additional information on the types of pattern variables that was previously hard to reconstruct (see #2151). When unquoting a clause, the types in the clause telescope are currently ignored (but this is subject to change in the future).

Three constructors of the Pattern datatype were also changed:
- pattern variables now refer to a de Bruijn index (relative to the clause telescope) rather than a string,
- absurd patterns take a de Bruijn index and are expected to be bound by the clause telescope,
- dot patterns now include the actual dotted term.
```
data Pattern where
  con    : (c : Name) (ps : List (Arg Pattern)) → Pattern
  dot    : (t : Term)    → Pattern   -- previously:   dot : Pattern
  var    : (x : Nat)     → Pattern   -- previously:   var : (x : String) → Pattern
  lit    : (l : Literal) → Pattern
  proj   : (f : Name)    → Pattern
  absurd : (x : Nat)     → Pattern
```
It is likely that this change to the reflected syntax requires you to update reflection code written for previous versions of Agda. Here are some tips for updating your code:
- When quoting a clause, you can recover the name of a pattern variable by looking up the given index in the clause telescope. The contents of dot patterns can safely be ignored (unless you have a use for them).
- When creating a new clause for unquoting, you need to create a telescope for the types of the pattern variables. To get back the old behaviour of Agda, it is sufficient to set all the types of the pattern variables to unknown. So you can construct the telescope by listing the names of all pattern variables and absurd patterns together with their ArgInfo. Meanwhile, the pattern variables should be numbered in order to update them to the new representation. As for the telescope types, the contents of a dot pattern can safely be set to unknown.
New operation in TC monad, execTC, which calls an external executable
```
execTC : (exe : String) (args : List String) (stdIn : String)
       → TC (Σ Nat (λ _ → Σ String (λ _ → String)))
```
The execTC builtin takes three arguments: the basename of the executable (e.g., "echo"), a list of arguments, and the contents of the standard input. It returns a triple, consisting of the exit code (as a natural number), the contents of the standard output, and the contents of the standard error.

The builtin is only available when --allow-exec is passed. (Note that --allow-exec is incompatible with --safe.) To make an executable available to Agda, add the absolute path on a new line in ~/.agda/executables.

Two new operations in the TC monad, onlyReduceDefs and dontReduceDefs:

onlyReduceDefs : ∀ {a} {A : Set a} → List Name → TC A → TC A
dontReduceDefs : ∀ {a} {A : Set a} → List Name → TC A → TC A

These functions allow picking a specific set of functions that should (resp. should not) be reduced while executing the given TC computation.

For example, the following macro unifies the current hole with the term 3 - 3:

macro₁ : Term -> TC ⊤
macro₁ goal = do
  u   ← quoteTC ((1 + 2) - 3)
  u'  ← onlyReduceDefs (quote _+_ ∷ []) (normalise u)
  unify u' goal

New operation in the TC monad, withReconstructed:

withReconstructed : ∀ {a} {A : Set a} → TC A → TC A

This function ensures reconstruction of hidden parameters after performing the TC computation. For example, consider the following type and function:

record RVec {a} (X : Set a) (n : Nat) : Set a where
  constructor vec
  field sel : Fin n → X

test-rvec : Nat → RVec Nat 5
test-rvec x = vec λ _ → x

In the reflected syntax the body of the test-rvec would be represented as con vec (unknown ∷ unknown ∷ unknown ∷ (lam _ x). The use of withReconstructed replaces unknowns with the actual values:

macro₂ : Name → Term → TC ⊤
macro₂ n hole = do
  (function (clause tel ps t ∷ [])) ←
    withReconstructed (getDefinition n)
    where _ → quoteTC "ERROR" >>= unify hole
  quoteTC t >>= unify hole

Three new constructors in the Sort datatype, prop : Level → Sort, propLit : Nat → Sort, and inf : Nat → Sort, representing the sorts Prop ℓ, Propᵢ, and Setωᵢ.
Terms that belong to a type in Prop are no longer unquoted to unknown but to a proper Term. (See #3553.)

Library management

.agda-lib files can now contain an extra field flags: with default flags for the library. Flags can be any flags that are accepted as part of an {-# OPTIONS ... #-} pragma. For example, file my-library.agda-lib with
```
flags: --without-K
```
will apply the --without-K flag to all Agda files in the current directory and (recursive) subdirectories that do not themselves contain an .agda-lib file.

Emacs mode

New command prefix C-u C-u C-u for weak-head normalization. For instance, given
```
downFrom : Nat → List Nat
downFrom 0 = []
downFrom (suc n) = n ∷ downFrom n
```
C-u C-u C-u C-c C-n downFrom 5 returns 4 ∷ downFrom 4.
New keyboard shortcut C-c C-x C-i for toggling display of irrelevant arguments.
One can no longer use commands like M-; (comment-dwim) to uncomment block comments. In return one can use M-; to comment out pragmas. (See #3329.)

JSON Interaction mode

Changes have been made to the structure of error and warning messages. The changes are summarized below. See #5052 for additional details.

The format of an error or warning was previously a bare string. Now, errors and warnings are represented by an object with a "message" key.

This means that responses previously structured like:
```
{"…": "…", "error": "Foo bar baz"}
```
will now be structured:
```
{"…": "…", "error": {"message": "Foo bar baz"}}
```
This applies directly to the PostPonedCheckFunDef response kind and Error info kind of the DisplayInfo response kind.
The format of collections of errors or warnings, which previously were each represented by a single newline-joined string, has been updated to represent each warning or error individually in a list.

That means that responses previously structured like:
```
{ "…": "…"
, "errors": "Postulates overcooked\nAxioms too wiggly"
, "warnings": "Something wrong\nSomething else\nwrong"
}
```
will now be structured:
```
{ "…": "…"
, "errors":
  [ { "message": "Postulates overcooked" }
  , { "message": "Axioms too wiggly" }
  ]
, "warnings":
  [ { "message": "Something wrong" }
  , { "message": "Something else\nwrong" }
  ]
}
```
This applies to CompilationOk, AllGoalsWarning, and Error info kinds of the DisplayInfo response kind.

The Error info kind of the DisplayInfo response kind has additionally been updated to distinguish warnings and errors.

An example of the previous format of a DisplayInfo response with an Error info kind was:

{
  "kind": "DisplayInfo",
  "info": {
    "kind": "Error",
    "message": "———— Error —————————————————————————————————————————————————\n/data/code/agda-test/Test.agda:2,1-9\nFailed to find source of module M in any of the following\nlocations:\n  /data/code/agda-test/M.agda\n  /data/code/agda-test/M.lagda\nwhen scope checking the declaration\n  import M\n\n———— Warning(s) ————————————————————————————————————————————\n/data/code/agda-test/Test.agda:3,1-10\nEmpty postulate block."
  }
}

The updated format is:

{
  "kind": "DisplayInfo",
  "info": {
    "kind": "Error",
    "error": {
      "message": "/data/code/agda-test/Test.agda:2,1-9\nFailed to find source of module M in any of the following\nlocations:\n  /data/code/agda-test/M.agda\n  /data/code/agda-test/M.lagda\nwhen scope checking the declaration\n  import M"
    },
    "warnings": [
      {
        "message": "/data/code/agda-test/Test.agda:3,1-10\nEmpty postulate block."
      }
    ]
  }
}

Compiler backends -----------------

With option --allow-unsolved-metas, code with holes can be compiled. If a hole is reached at runtime, the compiled program crashes. See issue #5103
Previously the GHC backend compiled at least one instance of Hinze's memoisation technique from "Memo functions, polytypically!" to reasonably efficient code. That is no longer the case (at least for that particular instance, see #5153).

LaTeX backend

The spacing in comments is now preserved when generating LaTex files from literate Agda. See #5320 for more details.

HTML backend

The named id attributes for local modules inside local modules are now different (see #5335).

For instance, consider the following Agda file:
```
module Top-level where

  module Inner where

    module Inside-inner where
```
Previously one could link to the module Inside-inner using a URL that ended with #Inside-inner. Now one can use #Inner.Inside-inner instead.

JS backend

Smaller local variable names in the generated JS code.

Previously: x0, x1, x2, ...

Now: a, b, c, ..., z, a0, b0, ..., z0, a1, b1, ...
Improved indentation of generated JS code.

More compact rendering of generated JS functions.

Previously:

exports["N"]["suc"] = function (x0) {
    return function (x1) {
      return x1["suc"](x0);
    };
  };

Now:

exports["N"]["suc"] = a => b => b["suc"](a);

Irrelevant arguments are now erased in the generated JS code.

Example Agda code:

flip : {A B C : Set} -> (B -> A -> C) -> A -> B -> C
flip f a b = f b a

Previously generated JS code:

exports["flip"] = function (x0) {
    return function (x1) {
      return function (x2) {
        return function (x3) {
          return function (x4) {
            return function (x5) {
              return x3(x5)(x4);
            };
          };
        };
      };
    };
  };

JS code generated now:

exports["flip"] = a => b => c => a(c)(b);

Record fields are not stored separately (the fields are stored only in the constructor) in the generated JS code.

Example Agda code:

record Sigma (A : Set) (B : A -> Set) : Set where
  field
    fst : A
    snd : B fst

Previously generated JS code (look at the "fst" and "snd" fields in the return value of exports["Sigma"]["record"]:

exports["Sigma"] = {};
exports["Sigma"]["fst"] = function (x0) {
    return x0["record"]({
      "record": function (x1, x2) {
        return x1;
      }
    });
  };
exports["Sigma"]["snd"] = function (x0) {
    return x0["record"]({
      "record": function (x1, x2) {
        return x2;
      }
    });
  };
exports["Sigma"]["record"] = function (x0) {
    return function (x1) {
      return {
        "fst": x0,
        "record": function (x2) {
          return x2["record"](x0, x1);
        },
        "snd": x1
      };
    };
  };

JS code generated now:

exports["Sigma"] = {};
exports["Sigma"]["fst"] = a => a["record"]({"record": (b,c) => b});
exports["Sigma"]["snd"] = a => a["record"]({"record": (b,c) => c});
exports["Sigma"]["record"] = a => b => ({"record": c => c["record"](a,b)});

--js-optimize flag has been added to the agda compiler.

With --js-optimize, agda does not wrap records in JS objects.

Example Agda code:

record Sigma (A : Set) (B : A -> Set) : Set where
  field
    fst : A
    snd : B fst

JS code generated without the --js-optimize flag:

exports["Sigma"] = {};
exports["Sigma"]["fst"] = a => a["record"]({"record": (b,c) => b});
exports["Sigma"]["snd"] = a => a["record"]({"record": (b,c) => c});
exports["Sigma"]["record"] = a => b => ({"record": c => c["record"](a,b)});

JS code generated with the --js-optimize flag:

exports["Sigma"] = {};
exports["Sigma"]["fst"] = a => a((b,c) => b);
exports["Sigma"]["snd"] = a => a((b,c) => c);
exports["Sigma"]["record"] = a => b => c => c(a,b);

With --js-optimize, agda uses JS arrays instead of JS objects. This is possible because constructor names are not relevant during the evaluation.

Example Agda code:

data Bool : Set where
  false : Bool
  true  : Bool

not : Bool -> Bool
not false = true
not true  = false

JS code generated without the --js-optimize flag:

exports["Bool"] = {};
exports["Bool"]["false"] = a => a["false"]();
exports["Bool"]["true"] = a => a["true"]();
exports["not"] = a => a({
    "false": () => exports["Bool"]["true"],
    "true": () => exports["Bool"]["false"]
  });

JS code generated with the --js-optimize flag:

exports["Bool"] = {};
exports["Bool"]["false"] = a => a[0/* false */]();
exports["Bool"]["true"] = a => a[1/* true */]();
exports["not"] = a => a([
    /* false */() => exports["Bool"]["true"],
    /* true */() => exports["Bool"]["false"]
  ]);

Note that comments are added to generated JS code to help human readers.

Erased branches are replaced by null in the generated array. If more than the half of branches are erased, the array is compressed to be a object like {3: ..., 13: ...}.

--js-minify flag has been added to the agda compiler.

With --js-minify, agda discards comments and whitespace in the generated JS code.

Agda as a library (API)

The SourceInfo record has been renamed to Source, and the sourceInfo function to parseSource.

Other issues

For 2.6.2, the following issues were also closed (see bug tracker):

#418: Unifier ignores presence of abstract keyword
#958: Module application display forms in parameterised modules
#1145: Allow multiple layout keywords on the same line
#2151: Add TC primitive to check left-hand side
#2461: Support with in the presence of IApply patterns
#2858: Feature request: Interleaving mutually-defined functions & datatypes
#3000: Interaction: iterated give encounters internal error
#3118: Feature request: default flags in .agda-lib file
#3289: Postfix projections should not have hiding information
#3360: Make Emacs mode available as a normal package via MELPA
#3365: Update GitHub linguist syntax highlight file
#3398: With the option --allow-unsolved-metas, the unsolved metas are not shown, only yellow
#3422: Show names of instance candidates in error message
#3486: Elaborate-and-give shouldn't reduce solution
#3532: Refine does not work for functions with 10 arguments or more
#3538: Regression: Rewrite rule involving constructors rejected in parametrized module
#3588: Refine suggests overloaded constructor which is not in scope
#3627: Where-blocks of clauses with irrelevant projections can use irrelevant variables
#3644: Error message without position
#3672: Better error messages for generalize easter eggs
#3684: Make error about non-existent record field a warning?
#3734: WARNING_ON_USAGE is not raised for constructors
#3744: Internal error related to abstract
#3870: Internal error during instance search
#3926: Document the effect of mutual to the order of type checking
#3933: import can remove definitions from scope
#3961: Missing documentation for coverage checking
#4071: Ill-scoped code in error message
#4088: Strange scoping rules for irrefutable with, part 2
#4093: Make it possible to rename Set?
#4109: cannot declare data types in Setω
#4131: Record definition doesn't compile without a specific let binding
#4132: The regular expression for floats in the lexer is too liberal
#4135: Constructor disambiguation picks non-unique solution
#4157: Agda gets confused by multiple anonymous definitions in a single mutual block
#4160: Printing implicit lambdas with --show-implicit
#4161: An alternative solution for hGetContent error on Windows when non-English
#4166: Instances that are not in scope are candidates for instance resolution
#4208: Field named _ in genTel record
#4252: Interaction ids get conflated after iterated give
#4265: Unsolved constraints when --no-syntactic-equality is used
#4280: Test case for #4169 fails in JS backend
#4291: Incorrect names can be generated for generalised variables
#4341: The documentation of inContext seems wrong.
#4350: Scoping bug with let open in telescope
#4365: Module fails to type check after parametrising it by postulates
#4400: TERMINATING pragma in where clause
#4425: [doc] What are .agdai files?
#4456: No error highlighting for error warnings?
#4458: The command agda2-measure-load-time is broken
#4481: Named implicit arguments do not behave the same in anonymous lambdas & definitions
#4482: "Unexpected implicit argument" should pinpoint exactly where the error starts
#4486: "did you mean" hint also for failing imports
#4491: Add a primitive for Data.Text's uncons
#4516: Internal error if files cannot be written to the directory for temporary files
#4518: Confusing error message if missing import
#4520: Better error for ambiguous BUILTIN [FROMNAT no longer working]
#4521: Massive increase in memory required to install Agda 2.6.1
#4526: Agda 2.6.1 bad performance: findProjectConfig slow in big directories
#4528: Internal error due to new forcing translation
#4530: Less normalization of goal types for instance search
#4534: [reflection] quote is not a defined name
#4536: co-pattern matching on empty record type removes body
#4538: changing the predefined precedence of an operator
#4543: Access violation on Windows on GHC 8.8.3, 8.8.4, 8.10.1 and 8.10.2
#4550: Documentation build: malformed code-block
#4556: [documentation] update hello world
#4557: activate github's discussions beta
#4560: Loss of canonicity with no-eta record and copatterns.
#4572: add PiSort and UnivSort to the documentation
#4576: quoteTC for Setω
#4580: No highlighting for pragmas FROMNAT, FROMNEG, FROMSTRING
#4583: QuickLaTeX backend does not highlight coinductive constructors as such
#4586: Better error message for "Not a valid let declaration"
#4593: The blocking machinery seems to be broken
#4595: Should Setω be a type?
#4596: Improve pattern matching on records in telescopes
#4606: The combination of Cubical Agda with inductive families is logically inconsistent
#4610: Support Emacs 27.1
#4615: Enable --no-sort-comparison by default?
#4621: Make --rewriting infective
#4623: Empty where blocks should get dead code warnings
#4631: Non-linear patterns handled in a buggy way
#4637: recCon-NOT-PRINTED in termination error in connection to with
#4638: Erased constructors
#4649: Repair Agda's REPL (agda -I) to work with --safe flag
#4656: Function name not wrapped in \AgdaFunction in generated LaTeX
#4662: Current module contents
#4665: Documentation: add install instructions for stack
#4671: Weird error message on case-insensitive file systems
#4679: Cubical: giving seems to skip the boundary condition check for extended lambdas
#4681: Get rid of auto-inlining?
#4684: Type error due to --no-syntactic-equality
#4687: Instance search fails with two equal candidates
#4704: Case-split generates invalid code
#4707: Just warn when using directive has repetitions
#4721: de Bruijn index out of scope when using rewriting
#4727: Meta-variable solutions contain subterms with the wrong modality
#4735: primShowQName creates not-in-scope names
#4737: Turn error Hiding ... has no effect into a warning
#4750: Unification failure in 2.6.1 and the master branch
#4752: Panic on unbound variable with pattern synonym
#4768: De Bruijn index @0 in error "Not a finite domain"
#4769: mergeEqualPs ignores Name and ArgInfo of merged-in patterns
#4772: C-u C-u C-c C-? should show all goals normalized (Cmd_metas)
#4773: Missing does-not-export warning for open directive for parametrised module
#4775: Internal error when trying to use incorrect lambda syntax to pattern match
#4784: Make erasure compatible with univalence
#4795: Build "agda-tests" fails using dynamic linking
#4815: Current master fails compilation: binding for 'error' shadows the existing binding
#4828: Symlinks are incorrectly followed during compilation
#4833: Internal error: cannot type-check file
#4851: BUILTIN SIGMA and --type-in-type
#4852: First load the file
#4857: Instance argument is printed as explicit argument
#4869: Internal error at src/full/Agda/TypeChecking/Serialise/Instances/Internal.hs:147
#4880: Non-dependent, irrelevant, nameless arguments aren't accepted in arrows
#4882: Missing reduce in literalStrategy
#4888: "Illegal declaration(s) before top-level module" in Agda 2.6.1
#4909: Rewrite rule not accepted with --no-fast-reduce
#4924: Instance resolution loops infinitely even when an instance is available
#4925: Too aggressive literal overloading
#4928: Internal error checking cubical library
#4929: Regression in 2.6.1 connected to forcing translation (internal error)
#4944: Generalize: stuck on constraint ↑ i =< ↑ (↑ i) : Size
#4946: Size polarity brittle with generalization
#4949: Cubical: internal error in eta-expansion under constraints
#4950: Range too large in complaint about missing definitions
#4951: Data types in Setω are treated as non-fibrant
#4952: Incorrect HTML generated for renaming clause
#4962: JS backend: bugs involving "null"
#4967: Crazy bug when defining Ord instances for Int
#4970: variable use adds explicit argument
#4975: "no such meta variable" when calling C-u C-c C-;
#4982: Internal error related to Cubical Agda
#4986: Pattern matching allows you to turn (x y : A) -> A into (@0 x y : A) -> A
#4995: No Cycle should not look under lambdas.
#4998: Make case in clause with instance projection does not work
#4999: primStringFromList is not injective because of surrogate code points
#5002: Bad JavaScript generated
#5005: Add flag to print AGDA_DIR and exit
#5029: One can override --safe
#5033: Internal error related to @tick
#5048: Disturbing names in normalised reflected type
#5064: Give more information in error "Pattern matching on no-eta record types is by default not allowed"
#5065: The termination checker is too liberal
#5079: Deep pattern-matching is sometimes allowed for erased arguments
#5093: Weird instance propagation between parameterised modules
#5112: make install-fix-whitespace shouldn't use the stack-X.Y.Z.yaml files used for Agda
#5128: getDefinition sometimes loses patterns
#5133: Current master fails LaTeX-related tests
#5140: test/LaTeXAndHTML/succeed contains failing tests
#5146: v2.6.1.2 does not contain MAlonzo/RTE/Float.hs
#5161: No error location for error in imported module when .agdai file exists
#5167: Fix broken compatibility with agda-bench
#5168: User manual: Missing instructions for installing Agda from Hackage using stack
#5176: mutual is deprecated in doc
#5204: Investigate highlighting failures
#5205: acmart examples in (user-manual) fail to build with latest TeXLive
#5207: Agda generated code does not type-check with GHC 9.0
#5210: Internal error
#5230: When stack.yaml exists, make calls stack, even on make debug
#5231: Problems compiling hello-world.agda
#5237: __IMPOSSIBLE__ from Agda.TypeChecking.Substitute
#5238: Rewrites are conjuring elements out of thin air
#5245: An infinite loop?
#5250: Change of warning options ignored
#5251: @0 annotation on lambda ignored
#5252: Internal error when case splitting pattern-lambda with higher rank type
#5286: Wrong error location with do notation and parse error in lhs
#5288: Very weird behaviour with compiled Data.Nat.Show.readMaybe
#5313: Documentation for internal level properties
#5314: Warn about abstract definitions without type signatures
#5317: The reflection machinery should support quantities
#5326: The highlighting code should be optimised
#5334: Meta-variable in constructor type busts interleaved mutual
#5335: Incorrect id attributes for local modules inside local modules
#5336: data Foo constructor {cs : ts} notation in interleaved mutual
#5339: constructor blocks do not tolerate overloading in same block
#5341: Do not make context variables non-erased
#5356: interleaved mutual: data _ where instead of constructor
#5358: tactic annotation on record field of function type drops domain in copattern definition
#5367: Parser regression involving with and let
#5370: Inconsistency in agda --help
#5375: Efficient conversion between interaction points and meta-variables
#5410: Module applications in where clauses of erased definitions yield non-erased code
#5419: Missing licences?
#5424: Internal error in v2.6.2 release candidate 1
#5434: The user manual's explanation of how erasure is checked for constructors does not match the implementation