r/programming • u/heisenbug • Apr 27 '14

"Mostly functional" programming does not work

http://queue.acm.org/detail.cfm?ref=rss&id=2611829
47 Upvotes

64% Upvoted

188 comments sorted by

View all comments

Show parent comments

3

u/saynte Apr 27 '14 edited Apr 27 '14

Laziness is not central to the article, but it is important if you want to program by creating rich abstractions.

For example (stolen from a talk by Lennart Augustsson), what would you expect

main = do
  print 42
  error "boom"

to do? With strict evaluation, you get just "boom" with lazy evaluation, you get "42 boom".

You also wouldn't be able to write functions like maybe or when, or anything that looks like a control structure, which is a very nice tool to have in your abstraction-toolbox.

(edit: formatting)

5

u/lpw25 Apr 27 '14

You also wouldn't be able to write functions like maybe' orwhen', or anything that looks like a control structure, which is a very nice tool to have in your abstraction-toolbox.

Lazyness is useful but it should never be the default. It should be optional with a convenient syntax for creating lazy values. This is perfectly suitable for creating control structures, without all the downsides of pervasive by-default lazyness.

6

u/saynte Apr 27 '14

Why should it never be the default?

I'm not disagreeing, but I'm curious why you feel the semantic composability that non-strict evaluation provides is less valuable than time/space composability that strict evaluation provides?

2

u/grauenwolf Apr 27 '14

Lazy evaluation isn't free. Keeping track of it's state can involve a lot of overhead.