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I have the following Haskell tuple:

       [("string",1,1)]

I need to extract the first element of this, obviously using 'fst' wont work here as there are 3 components.

What is the best method to use? sel ?

5 Answers5

18

You could just type your own function (we will use pattern matching) for this:

fst3 :: (a, b, c) -> a
fst3 (x, _, _) = x

and you use it like:

fst3 ("string", 1, 1)
Martinsos
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    @user2214957 If you don't know (I see you are new user): When you decide which answer you like the most, you should accept it as best/correct answer. – Martinsos Mar 28 '13 at 14:14
9

sel can be used in the following manner:

$ cabal install tuple
$ ghci
>>> :m +Data.Tuple.Select
>>> sel1 ("string",1,1)
"string"

It works like any other function with map

>>> map sel1 [("One",1,0),("Two",2,0),("Three",3,0)]
["One","Two","Three"]

The main advantage is that it works for a bigger tuple 

>>> sel1 ("string",1,1,1)
"string"

as well as the standard tuple 

>>> sel1 ("string",1)
"string"

hence there is no need of handling them separately.

Some more examples: 

>>> map sel2 [("One",1,0),("Two",2,0),("Three",3,0)]
[1,2,3]
(0.06 secs, 4332272 bytes)
>>> map sel3 [("One",1,0),("Two",2,0),("Three",3,0)]
[0,0,0]
(0.01 secs, 2140016 bytes)
>>> map sel4 [("One",1,0),("Two",2,0),("Three",3,0)]

<interactive>:6:5:
.... error
anaik
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zurgl
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    yes it is, for all tuple of size equal of less than 15 (as sel15 is defined but no sel16). – zurgl Mar 28 '13 at 14:23
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    this limitation is due to ghc tupe limitation for more information see http://stackoverflow.com/questions/2978389/haskell-tuple-size-limit – zurgl Mar 28 '13 at 14:25
  • I wouldn't consider `tuple` dead, just stable. – John L Mar 28 '13 at 16:15
  • Ok, I'd like to say no more maintained, Anyway I'm going to remove my consideration as it appears to be incorrect. – zurgl Mar 28 '13 at 16:21
  • Running `cabal install tuple` today (2021) gives me _Warning: You asked to install executables, but there are no executables in target: tuple. Perhaps you want to use --lib to install libraries instead._ – MEMark Jan 10 '21 at 07:34
5

You can also use the lens package:

> import Control.Lens
> Prelude Control.Lens> view _1 (1,2)  -- Or (1,2) ^. _1
1
> Prelude Control.Lens> view _1 (1,2,3) -- Or (1,2,3) ^. _1
1
> Prelude Control.Lens> view _1 (1,2,3,4) -- Or (1,2,3,4) ^. _1
1
> Prelude Control.Lens> view _1 (1,2,3,4,5) -- Or (1,2,3,4,5) ^. _1
1

This works for more than just the first element

> import Control.Lens
> Prelude Control.Lens> view _2 (1,2)  -- Or (1,2) ^. _2
2
> Prelude Control.Lens> view _3 (1,2,3) -- Or (1,2,3) ^. _3
3
> Prelude Control.Lens> view _4 (1,2,3,4) -- Or (1,2,3,4) ^. _4
4
> Prelude Control.Lens> view _5 (1,2,3,4,5) -- Or (1,2,3,4,5) ^. _5
5

I also wrote an answer to a similar question that covers more than just tuples: https://stackoverflow.com/a/23860744/128583

Community
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Davorak
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4

I would just define a function

fst3 :: (a,b,c) -> a
fst3 (x,_,_) = x

That's easy to understand and doesn't have a weird type (the type of sel1 is Sel1 a b => a -> b which may be confusing)

Or you could extract the value which you're interested in via patternmatching as in [x | (x,_,_) <- myThreeTupleList.

Finally, the best solution is to use a more structured data type! Surely, the string and the two ints carry more meaning and it's a good idea to encode that somehow...

yatima2975
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    @Martinsos: It's a pretty common name for this function; see for instance the [MissingH package](http://hackage.haskell.org/packages/archive/MissingH/1.1.1.0/doc/html/Data-Tuple-Utils.html). In fact, I think was supposed to be added to Data.Tuple in 2008: see [this message on the libraries mailing list](http://comments.gmane.org/gmane.comp.lang.haskell.libraries/8841). – yatima2975 Mar 28 '13 at 14:26
3

You could do this:

Prelude> let [(a,_,_)]=[("string",1,1)]
Prelude> a
"string"
su-
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