Allow constant values on Discriminated Unions

[pedromsantos]

Allow constant values on Discriminated Unions

I propose we allow discriminated unions to have constant values defined on the type

The existing way of approaching this problem in F# is creating a pattern match function to associate values with DU

Pros and Cons

The advantages of making this adjustment to F# are simplicity in design, specially in programs that make heavy usage of constant data.

Example using the famous Java planets sample (http://snipplr.com/view/42422/the-planet-enum-example/):

Using a tuple to associate constant values:

type Planets =
| MERCURY of (3.303e+23, 2.4397e6)
| VENUS of (4.869e+24, 6.0518e6)
| EARTH of (5.976e+24, 6.37814e6)
| MARS of (6.421e+23, 3.3972e6)
| JUPITER of (1.9e+27, 7.1492e7)
| SATURN of (5.688e+26, 6.0268e7)
| URANUS of (8.686e+25, 2.5559e7)
| NEPTUNE of (1.024e+26, 2.4746e7)

Using an anonymous record to associate constant values (if we get anonymous records in F#):

type Planets =
| MERCURY of {Mass=3.303e+23; Radius=2.4397e6)
| VENUS of {Mass=4.869e+24; Radius=6.0518e6),
| EARTH of {Mass=5.976e+24; Radius=6.37814e6),
| MARS of {Mass=6.421e+23; Radius=3.3972e6),
| JUPITER of {Mass=1.9e+27; Radius=7.1492e7),
| SATURN of {Mass=5.688e+26; Radius=6.0268e7),
| URANUS of {Mass=8.686e+25; Radius=2.5559e7),
| NEPTUNE of {Mass=1.024e+26; Radius=2.4746e7);

The disadvantages of making this adjustment to F# are apart form the work involved, I honestly don't know :)

Extra information

Estimated cost (XS, S, M, L, XL, XXL):

Related suggestions: (put links to reated suggestions here)

Affadavit (must be submitted)

Please tick this by placing a cross in the box:

• This is not a question (e.g. like one you might ask on stackoverflow) and I have searched stackoverflow for discussions of this issue
• I have searched both open and closed suggestions on this site and believe this is not a duplicate
• This is not something which has obviously "already been decided" in previous versions of F#. If you're questioning a fundamental design decision that has obviously already been taken (e.g. "Make F# untyped") then please don't submit it.

Please tick all that apply:

• This is not a breaking change to the F# language design
• I would be willing to help implement and/or test this
• I or my company would be willing to help crowdfund F# Software Foundation members to work on this