[Haskell-cafe] Type Coercion
tom.davie at gmail.com
Wed May 28 03:41:19 EDT 2008
On 28 May 2008, at 09:34, PR Stanley wrote:
> (16 :: Float) is a perfectly legitimate statement although I'm
> surprised that it's allowed in a type strong language such as
> Haskell. It's a bit like casting in good old C. What's going on here?
It's not a coercion -- it happens at compile time.
In a coercion, 16 starts off it's runtime life as an integer, gets a
couple of things done to it, and then is coerced into a floating point
number. What's happening here is you are telling the compiler "I
would like the number 16, but a floating point version of it please".
That instance of 16 always will have type Float.
Slightly more detail: numeric literals like this normally have the
type Int, but get pre-processed by the compiler. Whenever you write
16, the compiler writes (fromInteger 16). This has the effect that
instead of having the type Int, your literal has the type Num a => a.
You adding the type signature constrains it to being a Float again.
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