# 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 embedded DSL
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 use the type system to help write programs which
are *correct by construction*, interactively.

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, updated for Idris 2 compatibility as of 23 February 2021.

## Editing at the REPL

Note

The Idris2 repl does not support readline in the interest of
keeping dependencies minimal. Unfortunately this precludes some
niceties such as line editing, persistent history and completion.
A useful work around is to install rlwrap,
this utility provides all the aforementioned features simply by
invoking the Idris2 repl as an argument to the utility `rlwrap idris2`

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]
```

It is also possible to invoke Idris in a mode which runs a REPL command,
displays the result, then exits, using `idris2 --client`

. For example:

```
$ idris2 --client ':t plus'
Prelude.plus : Nat -> Nat -> Nat
$ idris2 --client '2+2'
4
```

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 c x`

command, abbreviated `:cs n c x`

, splits the
pattern variable `x`

on line `n`

at column `c`

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 12 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 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:

`\a`

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`

).`\s`

invokes a proof search to solve the hole under thecursor (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 `idris2 -–client`

.
More sophisticated support can be added by using the IDE protocol (yet to
be documented for Idris 2, but which mostly extends to protocol documented for
Idris 1.