I noticed a sort of surprising syntax thing in Elm yesterday. When you think about it, it’s not all that surprising, but the question is, can it be made of use? So what I noticed is that the in
expression of a let-in
expression can itself be a let-in
expression. So the following is valid syntax:
...
let
x = 1
in
let
y = 2
in
x + y
That’s because the syntax of let-in
is let <def-list> in <expr>
and let-in
is itself a valid <expr>
. So you can have as many let-in
blocks chained as you see fit. Clearly, you can just delete the middle two in-let
lines and you still have a valid expression. The question then is, what is this useful for?
Not much I guess, the only difference is that now definitions in the top-block cannot see any of the names defined in the bottom block, so whilst this is still valid:
...
let
x = 1
in
let
y = x + 2
in
x + y
The following is not valid, because y
is not in scope in the upper definition block:
...
let
x = 1 + y
in
let
y = 2
in
x + y
Could you use this? I’ve written before about the missing language feature to remove a name from the scope. It would be nice to be able to use this in such a fashion. That missing feature is to allow you to update the model without fear of accidentally using the old (non-updated) model. As in:
update message model =
case message of
SomeMessage ->
let
newModel =
... <expr> involving model
in
( { newModel | status = InFlight }
, requestEntries newModel
)
...
In this example the defined newModel
name is used in the in
expression twice, a common bug is to accidentally use model
where newModel
is correct. Unfortunately using multiple let-in
blocks chained does not help us solve this problem. I could possibly see a case where certain expressions need to be sure to use the old model
, rather than the new newModel
in which case chaining let-in
expressions might help with that. I think it’s a pretty marginal use though.