Instance Declaration Haskell

Question

My code is this:

type Code = [Inst]

data Inst = PUSH Int
          | PUSHV Name
          | POP Name
          | DO Op
          | JUMP Label
          | JUMPZ Label
          | LABEL Label
          deriving Show

doJump :: Inst -> Code -> Code
doJump l c = case (elemIndex l c) of
                 Just n -> drop (n+1) c
                 Nothing -> c

And GHC returns me this error, which has got me completely stumped... What I'm trying to do is return the list from the point after a specific element occurs.

No instance for (Eq Inst) arising from a use of `elemIndex'
Possible fix: add an instance declaration for (Eq Inst)
In the expression: (elemIndex l c)
In the expression:
  case (elemIndex l c) of {
    Just n -> drop (n + 1) c
    Nothing -> c }
In an equation for `doJump':
    doJump l c
      = case (elemIndex l c) of {
          Just n -> drop (n + 1) c
          Nothing -> c }

Any ideas?

Don't use tabs in Haskell. They don't work with significant indentation. — user1804599
– user1804599, Commented Apr 1, 2014 at 13:11
@rightfold There are arguments against using tabs in Haskell, but that isn't one of them. — Daniel Wagner
– Daniel Wagner, Commented Apr 1, 2014 at 13:20

user1804599 · Accepted Answer · 2014-04-01 13:09:10Z

5

elemIndex requires the elements of the list to be equality-comparable, so you need to add an instance for Eq Inst.

Use deriving Eq to have it generated automatically, for example:

data Inst = PUSH Int
          | PUSHV Name
          | POP Name
          | DO Op
          | JUMP Label
          | JUMPZ Label
          | LABEL Label
          deriving (Show, Eq)

answered Apr 1, 2014 at 13:09

user1804599

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3 Comments

stell1315 Over a year ago

Fantastic, that worked thanks. would it be possible to add an Instance of Eq Inst for just that function. Or must I do it for the whole data type?

Daniel Wagner Over a year ago

@stell1315 Instances are always program-global in Haskell.

Lee Duhem Over a year ago

@stell1315 If you do not want to add deriving Eq to Inst, you could use findIndex instead of elemIndex.

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