Proposal: Add singleton function to Data.List module

[Andreas Abel]

Sven Panne wrote:

 > Code is mainly *read*, not written, so the less you need
 > to know to read and understand code, the better it is.

Yes, and this is an argument *for* singleton, not against:

1. singleton is already known to the Haskell reader from the other 
collection types.

2. singleton is unambiguously invoking the image of creating a singleton 
collection.

In contrast, "pure" invokes the image of creating a effect-free monadic 
computation.  (:[]) invokes the image of a (mechanical) gorilla smiling 
at you. ;-)  All these associations distract from reading the code.  In 
both cases, a mental translation step is needed:

a) In the case of "pure", the conscious intervention that here we are 
not concerned with non-determinism (the list monad), but just creating a 
singleton collection.

b) In the case of "(:[])", the conscious intervention consists of
    - recognizing the decomposition (: [])
    - recognizing the operator section
    - understanding it as "cons something to the empty list"
    - understanding this as creating a singleton collection.

Both interventions hamper the flow of reading.

Of course, the procedure b) becomes automatic with use.  However, it is 
still a significant barrier to the newcomer.

Cheers,
Andreas

P.S.: if the gorilla (:[]) is vanishing from the Agda code base, I am 
mostly to be blamed for this.

On 2019-08-14 21:51, Sven Panne wrote:
> Am Mi., 14. Aug. 2019 um 21:35 Uhr schrieb Oliver Charles 
> <ollie at ocharles.org.uk <mailto:ollie at ocharles.org.uk>>:
> 
>     I'd also be interested in a kind of "reverse" argument - *if* we had
>     this added, what are the costs? There are arguments of "no,
>     pure/robot ninja is enough" - fine, we're not taking those away so
>     you aren't required to use singleton  But what is the *downside* of
>     having singleton? [...]
> 
> 
> A similar argument comes up with basically every extension, and it is a 
> very weak one. Code is mainly *read*, not written, so the less you need 
> to know to read and understand code, the better it is. Lots of different 
> ways to do the same thing makes programs harder to understand, and you 
> ultimately end up with Perl. :-}  And if we take this argument 
> seriously, we should probably add "doubleton" (synonym for "replicate 
> 2"), "tripleton" (synonym for "replicate 3"), etc., too.  Hey, you don't 
> have to use it after all, we're not taking away "replicate"...
> 
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