Surprise list
From Erights
Potentially surprising parts of the E language.
Current as of E 0.8.33o.
Contents |
For-loop pattern failure is not an error
If the key or value pattern of a for loop does not match, then the loop simply skips that element of the collection. This can be useful, but is unlike all other non-explicit pattern match operations (def and parameter lists).
Examples
# E sample
? def things := [1, "two", 3]
# value: [1, "two", 3]
? for x :int in things {
> println(x)
> }
# stdout: 1
# 3
#
Alternative
Move the pattern into a def:
# E sample
? for thing in things {
> def x :int := thing
> println(x)
> }
# stdout: 1
#
# problem: <ClassCastException: String doesn't coerce to an int>
The current developer version also allows strict checking to be turned on with a pragma (experimental):
# E sample
? pragma.enable("for-must-match")
? for x :int in things {
> println(x)
> }
# stdout: 1
#
# problem: <ClassCastException: String doesn't coerce to an int>
def x doesn't return x
The forward declaration expression, def var, does not return var but rather the Resolver for it.
Examples
# E sample ? def x # value: <Resolver> ? x # value: <Promise>
Rationale
If you want to pass a resolver as an argument, using it as an “out parameter”, this syntax is helpful:
x.hereIsAResolver(def y) ... use y
Alternative
If you want the actual promise, simply write (def x; x).
Unresolved references do not necessarily behave as their future resolved identity
— messages sent before the reference is resolved may be reacted to however the current holder of the reference “arrow-head” chooses, which does not necessarily correspond to the reference to which the unresolved reference resolves.
This has been discussed in an e-lang thread.
Rationale
This cannot be fixed without removing pipelining, eliminating one of the major benefits of the E reference model.
Examples
For now, see this lengthy example by MarkM.
Alternative
To avoid being vulnerable to this type of misbehavior, do not use a sameness test (==) or Map key lookup in order to decide on the reliability of the response to a previously sent message. This might involve using a when-catch/whenResolved construct to wait until the reference is resolved.
Accumulator operator is lowest-precedence
As you can see in the following expansion, the operator following the “_” is always lowest-precedence (as it is the one effectively rewritten into an assignment form):
? e`pragma.enable("accumulator"); accum 0 while (a) { _ * b + c }`
# value: e`null
...
# accum__1 := accum__1.multiply(b.add(c))
...
Rationale
None known; this is probably an accident of the definition of the expansion of accumulator syntax.
Alternative
Avoid accumulator syntax when the accumulation cannot be expressed as a single call.
Shadowing function arguments
Rebinding a name within a block is an error, e.g.
def foo() {
def x := 2
def x := 3
}
("Failed: x already in scope")
However, rebinding an argument does not issue any warning:
def foo(x) {
...
def x := 2
...
println(x)
}
Rationale
The already-in-scope error is intended to catch accidentally using the same name twice, not prohibit rebinding. Generally, you can expect it to be supressed anywhere there is visible {...} syntax.
Single-letter uriGetters are a special case
It is not possible to refer to a single-letter uriGetter in a URI literal.
? def <t> := ["foo" => "bar"]
# value: ["foo" => "bar"]
? interp.setExpand(true)
? <t:foo>
# expansion: file__uriGetter.get("t:foo")
# value: <file:/Users/kpreid/t:foo>
Rationale
This is a feature intended for convenient support for Windows drive letter filenames. XXX Look at whether MarkM agreed to remove it.
Alternative
Use names longer than one letter.
