Add convenience methods to read the value of float and int literals

[ahoppen]

I was trying to get the integer/float value of integer/float literals and was surprised to find there is no method for it and you need to parse the value from the characters in the digits token yourself. I feel like there should be a convenience method for that.

If you disagree and don't think we should increase the API surface for these kinds of methods, I‘m also happy with the decision.

[ahoppen]

@swift-ci Please test

[harlanhaskins]

Thanks for adding these.

[ahoppen]

@swift-ci Please test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - af07b60

[ahoppen]

@swift-ci Please test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 9b587ee

[rintaro]

Could you handle number literals with _? Currently Double/Int.init(_:String) returns nil for strings like "1_000".

[ahoppen]

@rintaro Ah. I didn’t think about that. Looking into the compiler we’d also need to support

1. Hexadecimal int literals
2. Octal int literals
3. Binary int literals
4. Exponential float literals (e.g. 5e+2)
5. Hexadecimal float literals with possible exponents (e.g. 0x5.fp+2)

1 - 3 should be fairly easy to implement using Int(_:radix:) but Swift is currently lacking corresponding initializers for 4 and 5. While I believe it would still be useful to have properties that are able to parse these (I’m sure nearly no developer will think about case 5), it’s out of scope for now.

I filed SR-11580 to keep track of the issue.