Equal 

LYAHFGG:

Eq is used for types that support equality testing. The functions its members implement are == and /=.

Scalaz equivalent for the Eq typeclass is called Equal:

scala> 1 === 1
res0: Boolean = true

scala> 1 === "foo"
<console>:14: error: could not find implicit value for parameter F0: scalaz.Equal[Object]
              1 === "foo"
              ^

scala> 1 == "foo"
<console>:14: warning: comparing values of types Int and String using `==' will always yield false
              1 == "foo"
                ^
res2: Boolean = false

scala> 1.some =/= 2.some
res3: Boolean = true

scala> 1 assert_=== 2
java.lang.RuntimeException: 1 ≠ 2

Instead of the standard ==, Equal enables ===, =/=, and assert_=== syntax by declaring equal method. The main difference is that === would fail compilation if you tried to compare Int and String.

Note: I originally had /== instead of =/=, but Eiríkr Åsheim pointed out to me:

@eed3si9n hey, was reading your scalaz tutorials. you should encourage people to use =/= and not /== since the latter has bad precedence.

— Eiríkr Åsheim (@d6) September 6, 2012

Normally comparison operators like != have lower higher precedence than &&, all letters, etc. Due to special precedence rule /== is recognized as an assignment operator because it ends with = and does not start with =, which drops to the bottom of the precedence:

scala> 1 != 2 && false
res4: Boolean = false

scala> 1 /== 2 && false
<console>:14: error: value && is not a member of Int
              1 /== 2 && false
                      ^

scala> 1 =/= 2 && false
res6: Boolean = false