# Interactive Editing¶

By now, we have seen several examples of how Idris’ dependent type
system can give extra confidence in a function’s correctness by giving
a more precise description of its intended behaviour in its *type*. We
have also seen an example of how the type system can help with EDSL
development by allowing a programmer to describe the type system of an
object language. However, precise types give us more than verification
of programs — we can also exploit types to help write programs which
are *correct by construction*.

The Idris REPL provides several commands for inspecting and modifying parts of programs, based on their types, such as case splitting on a pattern variable, inspecting the type of a hole, and even a basic proof search mechanism. In this section, we explain how these features can be exploited by a text editor, and specifically how to do so in Vim. An interactive mode for Emacs is also available.

## Editing at the REPL¶

The REPL provides a number of commands, which we will describe shortly, which generate new program fragments based on the currently loaded module. These take the general form

```
:command [line number] [name]
```

That is, each command acts on a specific source line, at a specific
name, and outputs a new program fragment. Each command has an
alternative form, which *updates* the source file in-place:

```
:command! [line number] [name]
```

When the REPL is loaded, it also starts a background process which
accepts and responds to REPL commands, using `idris --client`

. For
example, if we have a REPL running elsewhere, we can execute commands
such as:

```
$ idris --client ':t plus'
Prelude.Nat.plus : Nat -> Nat -> Nat
$ idris --client '2+2'
4 : Integer
```

A text editor can take advantage of this, along with the editing commands, in order to provide interactive editing support.

## Editing Commands¶

### :addclause¶

The `:addclause n f`

command (abbreviated `:ac n f`

) creates a
template definition for the function named `f`

declared on line
`n`

. For example, if the code beginning on line 94 contains:

```
vzipWith : (a -> b -> c) ->
Vect n a -> Vect n b -> Vect n c
```

then `:ac 94 vzipWith`

will give:

```
vzipWith f xs ys = ?vzipWith_rhs
```

The names are chosen according to hints which may be given by a programmer, and then made unique by the machine by adding a digit if necessary. Hints can be given as follows:

```
%name Vect xs, ys, zs, ws
```

This declares that any names generated for types in the `Vect`

family
should be chosen in the order `xs`

, `ys`

, `zs`

, `ws`

.

### :casesplit¶

The `:casesplit n x`

command, abbreviated `:cs n x`

, splits the
pattern variable `x`

on line `n`

into the various pattern forms it
may take, removing any cases which are impossible due to unification
errors. For example, if the code beginning on line 94 is:

```
vzipWith : (a -> b -> c) ->
Vect n a -> Vect n b -> Vect n c
vzipWith f xs ys = ?vzipWith_rhs
```

then `:cs 96 xs`

will give:

```
vzipWith f [] ys = ?vzipWith_rhs_1
vzipWith f (x :: xs) ys = ?vzipWith_rhs_2
```

That is, the pattern variable `xs`

has been split into the two
possible cases `[]`

and `x :: xs`

. Again, the names are chosen
according to the same heuristic. If we update the file (using
`:cs!`

) then case split on `ys`

on the same line, we get:

```
vzipWith f [] [] = ?vzipWith_rhs_3
```

That is, the pattern variable `ys`

has been split into one case
`[]`

, Idris having noticed that the other possible case ```
y ::
ys
```

would lead to a unification error.

### :addmissing¶

The `:addmissing n f`

command, abbreviated `:am n f`

, adds the
clauses which are required to make the function `f`

on line `n`

cover all inputs. For example, if the code beginning on line 94 is

```
vzipWith : (a -> b -> c) ->
Vect n a -> Vect n b -> Vect n c
vzipWith f [] [] = ?vzipWith_rhs_1
```

then `:am 96 vzipWith`

gives:

```
vzipWith f (x :: xs) (y :: ys) = ?vzipWith_rhs_2
```

That is, it notices that there are no cases for non-empty vectors, generates the required clauses, and eliminates the clauses which would lead to unification errors.

### :proofsearch¶

The `:proofsearch n f`

command, abbreviated `:ps n f`

, attempts to
find a value for the hole `f`

on line `n`

by proof search,
trying values of local variables, recursive calls and constructors of
the required family. Optionally, it can take a list of *hints*, which
are functions it can try applying to solve the hole. For
example, if the code beginning on line 94 is:

```
vzipWith : (a -> b -> c) ->
Vect n a -> Vect n b -> Vect n c
vzipWith f [] [] = ?vzipWith_rhs_1
vzipWith f (x :: xs) (y :: ys) = ?vzipWith_rhs_2
```

then `:ps 96 vzipWith_rhs_1`

will give

```
[]
```

This works because it is searching for a `Vect`

of length 0, of
which the empty vector is the only possibility. Similarly, and perhaps
surprisingly, there is only one possibility if we try to solve ```
:ps
97 vzipWith_rhs_2
```

:

```
f x y :: (vzipWith f xs ys)
```

This works because `vzipWith`

has a precise enough type: The
resulting vector has to be non-empty (a `::`

); the first element
must have type `c`

and the only way to get this is to apply `f`

to
`x`

and `y`

; finally, the tail of the vector can only be built
recursively.

### :makewith¶

The `:makewith n f`

command, abbreviated `:mw n f`

, adds a
`with`

to a pattern clause. For example, recall `parity`

. If line
10 is:

```
parity (S k) = ?parity_rhs
```

then `:mw 10 parity`

will give:

```
parity (S k) with (_)
parity (S k) | with_pat = ?parity_rhs
```

If we then fill in the placeholder `_`

with `parity k`

and case
split on `with_pat`

using `:cs 11 with_pat`

we get the following
patterns:

```
parity (S (plus n n)) | even = ?parity_rhs_1
parity (S (S (plus n n))) | odd = ?parity_rhs_2
```

Note that case splitting has normalised the patterns here (giving
`plus`

rather than `+`

). In any case, we see that using
interactive editing significantly simplifies the implementation of
dependent pattern matching by showing a programmer exactly what the
valid patterns are.

## Interactive Editing in Vim¶

The editor mode for Vim provides syntax highlighting, indentation and interactive editing support using the commands described above. Interactive editing is achieved using the following editor commands, each of which update the buffer directly:

`\d`

adds a template definition for the name declared on thecurrent line (using

`:addclause`

).

`\c`

case splits the variable at the cursor (using`:casesplit`

).

`\m`

adds the missing cases for the name at the cursor (using`:addmissing`

).

`\w`

adds a`with`

clause (using`:makewith`

).`\o`

invokes a proof search to solve the hole under thecursor (using

`:proofsearch`

).

`\p`

invokes a proof search with additional hints to solve thehole under the cursor (using

`:proofsearch`

).

There are also commands to invoke the type checker and evaluator:

`\t`

displays the type of the (globally visible) name under thecursor. In the case of a hole, this displays the context and the expected type.

`\e`

prompts for an expression to evaluate.`\r`

reloads and type checks the buffer.

Corresponding commands are also available in the Emacs mode. Support
for other editors can be added in a relatively straightforward manner
by using `idris –client`

.