xpct/docs/

writing_matchers.rs

/*!
# Writing Custom Matchers

How to write custom matchers for your tests.

[↩︎ Back to User Docs](crate::docs)

If none of the provided matchers suit your needs, xpct allows you to write
custom matchers. There are a few ways to do this. In increasing order of
complexity and flexibility, you can:

1. Compose existing matchers. This is the simplest approach, but doesn't let you
   customize the formatting of the failure output.
2. Implement [`Match`]. This lets you customize the formatting of the failure
   output.
3. Implement [`TransformMatch`]. This is like [`Match`], but additionally allows
   you to write matchers that transform values (like the [`be_some`] and
   [`be_ok`] matchers do).

## Composing existing matchers

The simplest way to make custom matchers is to just compose existing matchers.
The combinator matchers [`each`], [`any`], and [`all`] are useful for this.

```
use std::fmt;
use xpct::{each, be_lt, be_gt};
use xpct::core::Matcher;

pub fn be_between<'a, Actual, Low, High>(
    low: &'a Low,
    high: &'a High,
) -> Matcher<'a, Actual, Actual>
where
    Actual: PartialOrd<Low> + PartialOrd<High> + fmt::Debug + 'a,
    Low: fmt::Debug,
    High: fmt::Debug,
{
    each(move |ctx| {
        ctx.borrow().to(be_gt(low)).to(be_lt(high));
    })
}
```

## Implementing `Match`

The next simplest way is to implement the [`Match`] trait. This is how many of
the provided matchers are implemented. Here's an implementation of the [`equal`]
matcher.

```
use xpct::core::Match;
use xpct::matchers::Mismatch;

pub struct EqualMatcher<Expected> {
    expected: Expected,
}

impl<Expected> EqualMatcher<Expected> {
    pub fn new(expected: Expected) -> Self {
        Self { expected }
    }
}

impl<Expected, Actual> Match<Actual> for EqualMatcher<Expected>
where
    Actual: PartialEq<Expected> + Eq,
{
    type Fail = Mismatch<Expected, Actual>;

    fn matches(&mut self, actual: &Actual) -> xpct::Result<bool> {
        Ok(actual == &self.expected)
    }

    fn fail(self, actual: Actual) -> Self::Fail {
        Mismatch {
            actual,
            expected: self.expected,
        }
    }
}

```

Now let's make a function to call this matcher ergonomically from tests!
Basically, we just need to write a function which returns a [`Matcher`].

To make `EqualMatcher` into a `Matcher`, you just need to wrap it with
[`Matcher::new`]. This method also accepts the formatter which is used to format
the output. Thankfully, you don't need to write the formatting logic yourself to
get pretty output! Because our matcher returns a [`Mismatch`] when it fails, we
can use any formatter which accepts a [`Mismatch`], like the provided
[`MismatchFormat`].

```
# use xpct::matchers::equal::EqualMatcher;
use std::fmt;

use xpct::expect;
use xpct::core::Matcher;
use xpct::format::MismatchFormat;

pub fn equal<'a, Actual, Expected>(expected: Expected) -> Matcher<'a, Actual, Actual>
where
    Actual: fmt::Debug + PartialEq<Expected> + Eq + 'a,
    Expected: fmt::Debug + 'a,
{
    Matcher::new(
        EqualMatcher::new(expected),
        MismatchFormat::new("to equal", "to not equal"),
    )
}

```

What if we wanted to make a matcher which is the negated version of
`EqualMatcher`, like `not_equal`? For a matcher created by implementing
[`Match`], we can call [`Matcher::neg`] to negate it.

```
# use xpct::matchers::equal::EqualMatcher;
use std::fmt;

use xpct::expect;
use xpct::core::Matcher;
use xpct::format::MismatchFormat;

pub fn not_equal<'a, Actual, Expected>(expected: Expected) -> Matcher<'a, Actual, Actual>
where
    Actual: fmt::Debug + PartialEq<Expected> + Eq + 'a,
    Expected: fmt::Debug + 'a,
{
    Matcher::neg(
        EqualMatcher::new(expected),
        // Remember that we need to flip these cases, because `actual !=
        // expected` is now the *positive* case and `actual == expected` is now
        // the *negative* case.
        MismatchFormat::new("to not equal", "to equal"),
    )
}

expect!("disco").to(not_equal("not disco"));
```

## Implementing `TransformMatch`

The major limitation of [`Match`] is that it always returns the same value that
was passed in. If you need it to transform the value like the [`be_some`] and
[`be_ok`] matchers do, you can implement the [`TransformMatch`] trait.

```
use std::marker::PhantomData;

use xpct::core::{Matcher, TransformMatch, MatchOutcome};
use xpct::matchers::Expectation;

pub struct BeOkMatcher<T, E> {
    // Matchers created by implementing `TransformMatch` will often need to use
    // `PhantomData` so they know their input and output types.
    marker: PhantomData<(T, E)>,
}

impl<T, E> BeOkMatcher<T, E> {
    pub fn new() -> Self {
        Self {
            marker: PhantomData,
        }
    }
}

impl<T, E> TransformMatch for BeOkMatcher<T, E> {
    // The type the matcher accepts.
    type In = Result<T, E>;

    // In the positive case, this should return the `Ok` value.
    type PosOut = T;

    // In the negative case, this should return the `Err` value.
    type NegOut = E;

    // We use the `Expectation` type here to include the actual value in the
    // failure output.
    type PosFail = Expectation<Result<T, E>>;
    type NegFail = Expectation<Result<T, E>>;

    fn match_pos(
        self,
        actual: Self::In,
    ) -> xpct::Result<MatchOutcome<Self::PosOut, Self::PosFail>> {
        match actual {
            Ok(value) => Ok(MatchOutcome::Success(value)),
            Err(err) => Ok(MatchOutcome::Fail(Expectation { actual: Err(err) })),
        }
    }

    fn match_neg(
        self,
        actual: Self::In,
    ) -> xpct::Result<MatchOutcome<Self::NegOut, Self::NegFail>> {
        match actual {
            Ok(value) => Ok(MatchOutcome::Fail(Expectation { actual: Ok(value) })),
            Err(error) => Ok(MatchOutcome::Success(error)),
        }
    }
}
```

You'll see the terms "pos" and "neg", short for *positive* and *negative*,
throughout the API. These refer to whether a matcher is negated (negative)
or not negated (positive).

If a matcher is negated (the negative case), it means that we're expecting it to
fail. If a matcher is *not* negated (the positive case), it means we're
expecting it to succeed.

Now let's make some functions for invoking our matcher.

```
use std::fmt;

# use xpct::matchers::result::BeOkMatcher;
use xpct::core::{Matcher, NegFormat};
use xpct::format::ExpectationFormat;

// `ExpectationFormat` is a simple formatter that just returns the actual value
// and a static message.
fn result_format<T>() -> ExpectationFormat<T> {
    ExpectationFormat::new("to be Ok(_)", "to be Err(_)")
}

pub fn be_ok<'a, T, E>() -> Matcher<'a, Result<T, E>, T, E>
where
    T: fmt::Debug + 'a,
    E: fmt::Debug + 'a,
{
    // For matchers implemented with `TransformMatch`, you use
    // `Matcher::transform`.
    Matcher::transform(BeOkMatcher::new(), result_format())
}

pub fn be_err<'a, T, E>() -> Matcher<'a, Result<T, E>, E, T>
where
    T: fmt::Debug + 'a,
    E: fmt::Debug + 'a,
{
    // You can use `Matcher::transform_neg` to negate a matcher created by
    // implementing `TransformMatch`. You can use `NegFormat` to negate the
    // formatter.
    Matcher::transform_neg(BeOkMatcher::new(), NegFormat(result_format()))
}
```

[`TransformMatch`]: crate::core::TransformMatch
[`each`]: crate::each
[`any`]: crate::any
[`all`]: crate::all
[`be_some`]: crate::be_some
[`be_ok`]: crate::be_ok
[`Match`]: crate::core::Match
[`equal`]: crate::equal
[`Matcher`]: crate::core::Matcher
[`Matcher::transform`]: crate::core::Matcher::transform
[`Mismatch`]: crate::matchers::Mismatch
[`MismatchFormat`]: crate::format::MismatchFormat
[`Matcher::new`]: crate::core::Matcher::new
[`Matcher::neg`]: crate::core::Matcher::neg
*/