README.md

# Norm

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Norm is a system for specifying the structure of data. It can be used for
validation and for generation of data. Norm does not provide any set of
predicates and instead allows you to re-use any of your existing
validations.

```elixir
import Norm

conform!(123, spec(is_integer() and &(&1 > 0)))
=> 123

conform!(-50, spec(is_integer() and &(&1 > 0)))
** (Norm.MismatchError) val: -50 fails: &(&1 > 0)
    (norm) lib/norm.ex:44: Norm.conform!/2

user_schema = schema(%{
  user: schema(%{
    name: spec(is_binary()),
    age: spec(is_integer() and &(&1 > 0))
  })
})

input = %{user: %{name: "chris", age: 30, email: "c@keathley.io"}}

conform!(input, user_schema)
=> %{user: %{name: "chris", age: 30, email: "c@keathley.io"}}

user_schema
|> gen()
|> Enum.take(3)
=> [
  %{user: %{age: 0, name: ""}},
  %{user: %{age: 2, name: "x"}},
  %{user: %{age: 1, name: ""}}
]
```

Norm can also be used to specify contracts for function definitions:

```elixir
defmodule Colors do
  use Norm

  def rgb(), do: spec(is_integer() and &(&1 in 0..255))

  def hex(), do: spec(is_binary() and &String.starts_with?(&1, "#"))

  @contract rgb_to_hex(r :: rgb(), g :: rgb(), b :: rgb()) :: hex()
  def rgb_to_hex(r, g, b) do
    # ...
  end
end
```

## Validation and conforming values

Norm validates data by "conforming" the value to a specification. If the
values don't conform then a list of errors is returned. There are
2 functions provided for this `conform/2` and `conform!/2`. If you need to
return a list of well defined errors then you should use `conform/2`.
Otherwise `conform!/2` is generally more useful. The input data is
always passed as the 1st argument to `conform` so that calls to conform
are easily chainable.

### Predicates and specs

Norm does not provide a special set of predicates and instead allows you
to convert any predicate into a spec with the `spec/1` macro. Predicates
can be composed together using the `and` and `or` keywords. You can also
use anonymous functions to create specs.

```elixir
spec(is_binary())
spec(is_integer() and &(&1 > 0))
spec(is_binary() and fn str -> String.length(str) > 0 end)
```

The data is always passed as the first argument to your predicate so you
can use predicates with multiple values like so:

```elixir
def greater?(x, y), do: x > y
conform!(10, spec(greater?(5)))
=> 10
conform!(3, spec(greater?(5)))
** (Norm.MismatchError) val: 3 fails: greater?(5)
    (norm) lib/norm.ex:44: Norm.conform!/2
```

### Tuples and atoms

Atoms and tuples can be matched without needing to wrap them in a function.

```elixir
:atom = conform!(:atom, :atom)

{1, "hello"} = conform!({1, "hello"}, {spec(is_integer()), spec(is_binary())})

conform!({1, 2}, {:one, :two})
** (Norm.MismatchError) val: 1 in: 0 fails: is not an atom.
val: 2 in: 1 fails: is not an atom.
```

Because Norm supports matching on bare tuples we can easily validate functions
that return `{:ok, term()}` and `{:error, term()}` tuples.

```elixir
# if User.get_name/1 succeeds it returns {:ok, binary()}
result = User.get_name(123)
{:ok, name} = conform!(result, {:ok, spec(is_binary())})
```

These specifications can be combined with `one_of/1` to create union types.

```elixir
result_spec = one_of([
  {:ok, spec(is_binary())},
  {:error, spec(fn _ -> true end)},
])

{:ok, "alice"} = conform!(User.get_name(123), result_spec)
{:error, "user does not exist"} = conform!(User.get_name(-42), result_spec)
```

### Collections

Norm can define collections of values using `coll_of`.

```elixir
[1, 2, 3] = conform!([1,2,3], coll_of(spec(is_integer)))
```

Collections can take a number of options:

* `:kind` - predicate function the kind of collection being conformed
* `:distinct` - boolean value for specifying if the collection should have distinct elements
* `:min_count` - Minimum element count
* `:max_count` - Maximum element count
* `:into` - The output collection the input will be conformed into. If not specified then the input type will be used.

```elixir
conform!([:a, :b, :c], coll_of(spec(is_atom), into: MapSet.new()))
# => #MapSet<[:a, :b, :c]>
```

### Schemas

Norm provides a `schema/1` function for specifying maps and structs:

```elixir
user_schema = schema(%{
  user: schema(%{
    name: spec(is_binary()),
    age: spec(is_integer()),
  })
})

conform!(%{user: %{name: "chris", age: 31}}, user_schema)
=> %{user: %{name: "chris", age: 31}}

conform!(%{user: %{name: "chris", age: -31}}, user_schema)
** (Norm.MismatchError) in: :user/:age val: -31 fails: &(&1 > 0)
    (norm) lib/norm.ex:44: Norm.conform!/2
```

Schema's are designed to allow systems to grow over time. They provide this
functionality in two ways. The first is that any unspecified fields in the input
are passed through when conforming the input. The second is that all keys in a
schema are optional. This means that all of these are valid:

```elixir
user_schema = schema(%{
  name: spec(is_binary()),
  age: spec(is_integer()),
})

conform!(%{}, user_schema)
=> %{}
conform!(%{age: 31}, user_schema)
=> %{age: 31}
conform!(%{foo: :foo, bar: :bar}, user_schema)
=> %{foo: :foo, bar: :bar}
```

If you're used to more restrictive systems for managing data these might seem
like odd choices. We'll see how to specify required keys when we discuss Selections.

#### Structs

You can also create specs from structs:

```elixir
defmodule User do
  defstruct [:name, :age]

  def s, do: schema(%__MODULE__{
      name: spec(is_binary()),
      age: spec(is_integer())
    })
end
```

This will ensure that the input is a `User` struct with the key that match
the given specification. Its convention to provide a `s()` function in the
module that defines the struct so that schema's can be shared throughout
your system.

You don't need to provide specs for all the keys in your struct. Only the
specced keys will be conformed. The remaining keys will be checked for
presence.

```elixir
defmodule User do
  defstruct [:name, :age]
end

conform!(%User{name: "chris"}, schema(%User{}))
=> %User{name: "chris", age: nil}
```

#### Key semantics

Atom and string keys are matched explicitly and there is no casting that
occurs when conforming values. If you need to match on string keys you
should specify your schema with string keys.

Schemas accomodate growth by disregarding any unspecified keys in the input map.
This allows callers to start sending new data over time without coordination
with the consuming function.

### Selections and optionality

We said that all of the fields in a schema are optional. In order to specify
the keys that are required in a specific use case we can use a Selection. The
Selections takes a schema and a list of keys - or keys to lists of keys - that
must be present in the schema.

```elixir
user_schema = schema(%{
  user: schema(%{
    name: spec(is_binary()),
    age: spec(is_integer()),
  })
})
just_age = selection(user_schema, [user: [:age]])

conform!(%{user: %{name: "chris", age: 31}}, just_age)
=> %{user: %{age: 31, name: "chris"}}

conform!(%{user: %{name: "chris"}}, just_age)
** (Norm.MismatchError) Could not conform input:
val: %{name: "chris"} in: :user/:age fails: :required
    (norm) lib/norm.ex:387: Norm.conform!/2
```

If you need to mark all fields in a schema as required you can elide the list
of keys like so:

```elixir
user_schema = schema(%{
  user: schema(%{
    name: spec(is_binary()),
    age: spec(is_integer()),
  })
})

# Require all fields recursively
conform!(%{user: %{name: "chris", age: 31}}, selection(user_schema))
```

Selections are an important tool because they give control over optionality
back to the call site. This allows callers to determine what they actually need
and makes schema's much more reusable.

### Patterns

Norm provides a way to specify alternative specs using the `alt/1`
function. This is useful when you need to support multiple schema's or
multiple alternative specs.

```elixir
create_event = schema(%{type: spec(&(&1 == :create))})
update_event = schema(%{type: spec(&(&1 == :update))})
event = alt(create: create_event, update: update_event)

conform!(%{type: :create}, event)
=> {:create, %{type: :create}}

conform!(%{type: :update}, event)
=> {:update, %{type: :update}}

conform!(%{type: :delete}, event)
** (Norm.MismatchError)
  in: :create/:type val: :delete fails: &(&1 == :create)
  in: :update/:type val: :delete fails: &(&1 == :update)
```

## Generators

Along with validating that data conforms to a given specification, Norm
can also use specificiations to generate examples of good data. These
examples can then be used for property based testing, local development,
seeding databases, or any other use case.

```elixir
user_schema = schema(%{
  user: schema(%{
    name: spec(is_binary()),
    age: spec(is_integer() and &(&1 > 0))
  })
})
conform!(%{user: %{name: "chris", age: 30}}, user_schema)
=> %{user: %{name: "chris", age: 30}}

user_schema
|> gen()
|> Enum.take(3)
=> [
  %{user: %{age: 0, name: ""}},
  %{user: %{age: 2, name: "x"}},
  %{user: %{age: -2, name: ""}}
]
```

Under the hood Norm uses StreamData for its data generation. This means
you can use your specs in tests like so:

```elixir
input_data = schema(%{"user" => schema(%{"name" => spec(is_binary())})})

property "users can update names" do
  check all input <- gen(input_data) do
    assert :ok == update_user(input)
  end
end
```

### Built in generators

Norm will try to infer the generator to use from the predicate defined in
`spec`. It looks specifically for the guard clauses used for primitive
types in elixir. Not all of the built in guard clauses are supported yet.
PRs are very welcome ;).

### Guiding generators

You may have specs like `spec(fn x -> rem(x, 2) == 0 end)` which check to
see that an integer is even or not. This generator expects integer values
but there's no way for Norm to determine this. If you try to create
a generator from this spec you'll get an error:

```elixir
gen(spec(fn x -> rem(x, 2) == 0 end))
** (Norm.GeneratorError) Unable to create a generator for: fn x -> rem(x, 2) == 0 end
    (norm) lib/norm.ex:76: Norm.gen/1
```

You can guide Norm to the right generator by specifying a guard clause as
the first predicate in a spec. If Norm can find the right generator then
it will use any other predicates as filters in the generator.

```elixir
Enum.take(gen(spec(is_integer() and fn x -> rem(x, 2) == 0 end)), 5)
[0, -2, 2, 0, 4]
```

But its also possible to create filters that are too specific such as
this:

```elixir
gen(spec(is_binary() and &(&1 =~ ~r/foobarbaz/)))
```

Norm can determine the generators to use however its incredibly unlikely
that Norm will be able to generate data that matches the filter. After 25
consecutive unseccessful attempts to generate a good value Norm (StreamData
under the hood) will return an error. In these scenarios we can create
a custom generator.

### Overriding generators

You'll often need to guide your generators into the interesting parts of the
state space so that you can easily find bugs. That means you'll want to tweak
and control your generators. Norm provides an escape hatch for creating your
own generators with the `with_gen/2` function:

```elixir
age = spec(is_integer() and &(&1 >= 0))
reasonable_ages = with_gen(age, StreamData.integer(0..105))
```

Because `gen/1` returns a StreamData generator you can compose your generators
with other StreamData functions:

```elixir
age = spec(is_integer() and &(&1 >= 0))
StreamData.frequency([
  {3, gen(age)},
  {1, StreamData.binary()},
])

gen(age) |> StreamData.map(&Integer.to_string/1) |> Enum.take(5)
["1", "1", "3", "4", "1"]
```

This allows you to compose generators however you need to while keeping your
generation co-located with the specification of the data.

## Adding contracts to functions

You can `conform` data wherever it makes sense to do so in your application.
But one of the most common ways to use Norm is to validate a functions arguments
and return value. Because this is such a common pattern, Norm provides function
annotations similar to `@spec`:

```elixir
defmodule Colors do
  use Norm

  def rgb(), do: spec(is_integer() and &(&1 in 0..255))

  def hex(), do: spec(is_binary() and &String.starts_with?(&1, "#"))

  @contract rgb_to_hex(r :: rgb(), g :: rgb(), b :: rgb()) :: hex()
  @doc "Convert an RGB value to its CSS-style hexadecimal notation."
  def rgb_to_hex(r, g, b) do
    # ...
  end
end
```

If the arguments for `rgb_to_hex` don't conform to the specification or if
`rgb_to_hex` does not return a value that conforms to `hex` then an error will
be raised.

Note `@contract` must be placed _before_ `@doc` as above for `ExDoc` and
`ExUnit.DocTest` to continue working as intended.

<!-- MDOC !-->

## Installation

Add `norm` to your list of dependencies in `mix.exs`. If you'd like to use
Norm's generator capabilities then you'll also need to include StreamData
as a dependency.

```elixir
def deps do
  [
    {:stream_data, "~> 0.4"},
    {:norm, "~> 0.13"}
  ]
end
```

## Should I use this?

Norm is still early in its life so there may be some rough edges. But
we're actively using this at my current company (Bleacher Report) and
working to make improvements.

## Contributing and TODOS

Norm is being actively worked on. Any contributions are very welcome. Here is a
limited set of ideas that are coming soon.

- [ ] More streamlined specification of keyword lists.
- [ ] Support "sets" of literal values
- [ ] specs for functions and anonymous functions
- [ ] easier way to do dispatch based on schema keys