It's not clear to me why Elm uses `let`, instead of simply scoping
definitions to the expression below them.
With `let`:
foo =
let
a = 1
b = 2
in
a + b
Scoping definitions to the expression below them:
foo =
a = 1
b = 2
a + b
I understand that each function must contain a single expression. In Elm,
although they contain expressions, definitions are not expressions.
Visualized:
foo =
<EXPRESSION HERE>
foo =
a + 2 <- EXPRESSION
foo =
a = 1 <- DEFINITION SCOPED TO THE a + 2 EXPRESSION
a + 2
Another way to demonstrate scope is:
let
a = 1
b = 2
in
a + b
would become (parenthesis to demonstrate scope):
(
a = 1
b = 2
a + b
)
It seems to me that `let` and `in` are unnecessary and verbose. Put another
way, I think few people would agree that requiring a keyword before
variable assignment `set a = 1` would be a good idea. The `=` makes the
intent explicit. Likewise, indentation—or parenthesis—could make scopes
explicit, and `let` and `in` unnecessary.
Some have argued that without `let`, we could not have arbitrarily nested
scopes. I don't have significant experience with Elm, but I would guess
that nesting `let`s today is pretty big code smell. Instead of nesting
`let`s to reuse variable names, developers should either pick more
descriptive variable names, or abstract into a function.
This could—of course—apply anywhere an expression is expected:
True ->
x = 0
y = 0
(x, y)
...
@rtfeldman on the Slack pointed out that this syntax is more diff friendly:
if I write a view function like
view model =
div []
[ ... lots of other stuff ]
and then I want to introduce a nested constant like so:
view model =
let
foo = ...
in
div []
[ ... lots of other stuff ]
the fact that I indented the final expression makes the VCS diff explode
this happens to me all the time, and it's pretty annoying
with [this] idea it wouldn't happen anymore
Lastly, here's elm-todomvc with scoped definitions, courtesy of @rtfeldman
again:
https://github.com/rtfeldman/elm-todomvc/blob/8678c8bcaeb5cb4b3f87dbefb7a01b5fe492dbc7/Todo.elm
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