Haskell rare pattern matching

Question

In haskell, I can do:

s@"Hello" = s 

The result is:

>s
>"

It begins to print the String but never ends, what is goin on?

Answer 1

The @"Hello" is irrelevant here, all it does is fix the type to String. You get the same behaviour with

s :: String
s = s

Which is semantically equivalent to

s' :: String
s' = undefined

which gives the result

Prelude> s'
"*** Exception: Prelude.undefined

What I mean by “semantically equivalent” is, both s and s' are examples of bottom values, i.e. values from that “sin bin of error values any type contains, thanks to non-strictness”. As soon as you hit a bottom value, the pure Haskell language is basically powerless and has give in to, well, undefined, “impure behaviour”, like letting you wait forever or throwing an exception.

However, again thanks to non-strictness, this needs not necessarily happen. When printing a value, the first thing that happens is, the Show instance is invoked and asked to produce a string. A Haskell string is a lazy list. And the show of any string begins with ", therefore even if the string itself is utterly undefined, show will manage to produce that one character.

We can observe this more sheltered with

Prelude> head $ show s
'"'