The standard counter-argument is that by enforcing purity, Haskell improves encapsulation and the ability to reason about the program (see paeans about equation

The standard counter-argument is that by enforcing purity, Haskell improves encapsulation and the ability to reason about the program (see paeans about equational reasoning [1]), and prevents all the myriad classes of bugs associated with mutable state - thus greatly improving productivity. (The same applies to its more precise type system in general.) I'm not sure how much I believe it, but some people swear by it... [1] http://www.haskellforall.com/2013/12/equational-reasoning.html http://www.haskellforall.com/2013/12/equational-reasoning.ht...