Hidden features of Haskell [closed]

Answer 1

Equational Reasoning

Haskell, being purely functional allows you to read an equal sign as a real equal sign (in the absence of non-overlapping patterns).

This allows you to substitute definitions directly into code, and in terms of optimization gives a lot of leeway to the compiler about when stuff happens.

A good example of this form of reasoning can be found here:

http://www.haskell.org/pipermail/haskell-cafe/2009-March/058603.html

This also manifests itself nicely in the form of laws or RULES pragmas expected for valid members of an instance, for instance the Monad laws:

1. returrn a >>= f == f a
2. m >>= return == m
3. (m >>= f) >>= g == m >>= (\x -> f x >>= g)

can often be used to simplify monadic code.