Can someone explain me how I should understand Semigroup?

[Munkyu-Yang]

fantasy-land/concat :: Semigroup a => a ~> a -> a

Since there are so many as, it's quite confusing though I've read the documentation. Can someone provide me an actual example please?

[davidchambers]

Here are some examples, using S.concat:

> S.concat ('abc') ('def')
'abcdef'

> S.concat ([1, 2, 3]) ([4, 5, 6])
[1, 2, 3, 4, 5, 6]

> S.concat ({x: 1, y: 2}) ({y: 3, z: 4})
{x: 1, y: 3, z: 4}

> S.concat (S.Just ([1, 2, 3])) (S.Just ([4, 5, 6]))
Just ([1, 2, 3, 4, 5, 6])

> S.concat (Sum (18)) (Sum (24))
Sum (42)

S.concat is easier to understand than the underlying fantasy-land/concat methods for several reasons:

• its type signature does not contain a squiggly arrow (~>);
• it does not reference fantasy-land/concat explicitly; and
• it works for some built-in types such as String, even though String.prototype['fantasy-land/concat'] is undefined.

Here is an example of invoking a fantasy-land/concat method directly:

> S.Just ('abc') ['fantasy-land/concat'] (S.Just ('def'))
Just ('abcdef')

[CrossEye]

As well ad David's excellent answer, Tom Harding's series on these FantasyLand types is excellent. Semigroups is the fourth entry.

[djnzx]

Just a Monoid without zero value. Operation only. f: (A, A) => A

[juboba]

fantasy-land/concat :: Semigroup a => a ~> a -> a

Since there are so many as, it's quite confusing though I've read the documentation. Can someone provide me an actual example please?

If you want to understand the type signature, reading from left to right:

• Semigroup a => means: "The a that you will see from now on has a Semigroup"
• a ~> means: The following function is a method of an a (concat is a method of an instance that has a Semigroup)
• a -> a means: it's a function that receives an a and returns something of the same type a

In David's last example the first S.Just ('abc') is the a before the ~>.

[CrossEye]

@jujoba: There's a typo in the second bullet.

• a ~> means: The following function is a method of an a (chain is a method of an instance that has a Semigroup)

should read

• a ~> means: The following function is a method of an a (concat is a method of an instance that has a Semigroup)