Can I automatically generate a list of data values in Haskell?

Question

Let's say I create an enumerable data set (or two) in Haskell.

data Note = C | CsDb | D | DsEb | E | F | FsGb | G | GsAb | A | AsBb | B deriving (Read, Show, Eq, Ord)
data Diatonic = Unison | Min2 | Maj2 | Min3 | Maj3 | Per4 | Tritone | Per5 | Min6 | Maj6 | Min7 | Maj7 | Octave deriving (Read, Show, Eq, Ord)

I'd like to have access to a list of these values, and this is the only way I know how:

notes :: [Note]
notes = [C, CsDb, D, DsEb, E, F, FsGb, G, GsAb, A, AsBb, B]

diatonics :: [Diatonic]
diatonics = [Unison, Min2, Maj2, Min3, Maj3, Per4, Tritone, Per5, Min6, Maj6, Min7, Maj7, Octave]

This seems like redundant boilerplate. Is this the only way to create such a list, or can Haskell do it for me somehow?

note that this is not a list of data-types - it's still a list of values - maybe you should change the title — Random Dev
– Random Dev, Commented Nov 24, 2015 at 9:02

Bartek Banachewicz · Accepted Answer · 2015-11-24 09:16:32Z

12

if you add automatic deriving for Enum and Bounded it's as easy as:

data Note = C | CsDb | D | DsEb | E | F | FsGb | G | GsAb | A | AsBb | B
          deriving (Read, Show, Eq, Ord, Enum, Bounded)

notes :: [Note]
notes = [minBound .. maxBound]

example:

λ> notes
[C,CsDb,D,DsEb,E,F,FsGb,G,GsAb,A,AsBb,B]

I'll guess you will know how to do the other ;)

you need the Enum for the [ .. ] syntax and Bounded for minBound and maxBound - so you don't have to use Bounded if you don't want but in this case you have to add the bounds yourself:

data Note = C | CsDb | D | DsEb | E | F | FsGb | G | GsAb | A | AsBb | B
          deriving (Read, Show, Eq, Ord, Enum)

notes :: [Note]
notes = [C .. B]

edited Nov 24, 2015 at 9:16

Bartek Banachewicz

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answered Nov 24, 2015 at 8:57

Random Dev

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