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Type classes are for reusability

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I see a certain overuse of typeclasses in Haskell libraries.

Type classes are not intended for massive re-use of some short identifier like
fmap
or in order to access some syntactic sugar

(e.g. do notation or number literals). Instead type classes exist for writing reusable code.

Functions written using the
Num
type class like polynomial multiplication work (almost) equally for
Int
,
Integer
,
Rational
,
Float
,
Double
and custom number types. Functions like
mapM
and
replicateM
that only need a
Monad
constraint can be used for all monads including any combination of monad transformers.

Thus, think twice:

Do you really need a
Num
instance for a network port, just in order to be able to write
80
for a port number? How would sensible definitions of
(*)
would look like? (Actually, it was not necessary to bundle the number literal feature expressed by the
fromInteger
method with the
(+)
and
(*)
operations,

and indeed Numeric Prelude chooses a different set of operations. But it is generally accepted that number literals are reserved for objects that allow some sort of arithmetics.)

Isn't
port 80
even more comprehensible? Do you really need a
Functor
instance for the pair type?

What distinguishes the second member from the first member? Is the generic pair type the right choice, or should it better be a custom type?

Do you really need a
Monad
instance for writing something to a stream?

Or did you define the instance only for the do notation? Please, give the monoid a try!


See also