[Haskell-beginners] Syntax of Complex

[Bob Ippolito]

On Monday, June 15, 2015, Shishir Srivastava <shishir.srivastava at gmail.com>
wrote:

> Hi,
>
> The Data.Complex package defines the new data type 'Complex' as
>
> ----------
>
> data Complex a  = !a :+ !a
>
> -------
>
> Where ':+' is an infix operator. I don't however understand the usage of
> '!' in front of the type variable 'a'. What exactly is the purpose of '!' ?
>

'!' is a strictness annotation. This means that when the value is forced to
weak-head normal form (by pattern matching, BangPatterns, or seq), then the
fields with the ! will also be forced to weak-head normal form.

See also:
http://chimera.labs.oreilly.com/books/1230000000929/ch02.html#sec_par-eval-whnf
https://hackhands.com/lazy-evaluation-works-haskell/

-bob
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