E08097: assign-if-unset ':=?' always applies because the target is always unset here - it is a plain assignment; use ':=' (or declare the variable with the value)

What went wrong

An assign-if-unset ':=?' is applied to a target that is always unset at this point, so it always assigns: it is a plain assignment written as a conditional one. Data flow analysis proves the target unset here - it was given a value made by a no-argument constructor such as 'String()' or 'Integer()', or this code is inside 'if not x?' - so the "only if unset" part never decides anything.

Worked examples

Compilable examples demonstrating this error and the correct fix:

ek9 -q 2254 (Why is 'name <- String()' then 'name :=? "Steve"' reported...)
ek9 -q 1263 (If I have a String variable that might be unset, how can I...)
ek9 -q 1264 (Walk me through using :=? on an Integer retry counter so th...)
ek9 -q 2255 (How does the EK9 compiler decide a variable is definitely...)

Rationale

':=?' exists for a value that MAY already be set: it keeps the value if there is one and supplies a fallback if not. Used on a value that can only be unset, it says something the code does not mean, and a reader looks for the case where it does not assign. It is the mirror of E08095, where the target is always SET and ':=?' can never apply: between them, a ':=?' the compiler can decide is an error either way.

The head of a fallback chain is the usual case. 'x <- String()' then 'x :=? a()' then 'x :=? b()' starts from a value that is known to be unset, so the first ':=?' always applies. Start the chain from its first source instead - 'x <- a()' then 'x :=? b()'.

This is reported only when the compiler is SURE. EK9 has no warnings, so an error must be certain: a value that came from a function call may be set, and the compiler stays silent; inside a loop the value must be unset on every pass. Three places are never reported. A variable, returning variable or property declared with NO value ('x as String?') holds no value at all until it is first written, and ':=?' is the form that fills it - the case it exists for. In a 'pure' scope reassignment is not allowed (E08100), so ':=?' is the only way to give a value there. A guard such as 'if x :=? f()' also gates its block, which ':=' would not.

How to fix it

Use ':=' - it says what the statement does.
Or declare the variable with the value in the first place, instead of declaring it unset

and assigning it on the next line.

For a fallback chain, declare the variable from the first source and keep ':=?' for the

rest.

Common causes

'name <- String()' then 'name :=? "default"' - write 'name <- "default"'
A fallback chain that starts from 'String()' or 'Integer()' instead of its first source
A returning variable declared '<- rtn as String: String()' and then filled with ':=?'
Giving a value with ':=?' inside 'if not x?', the one place it is known to be unset

Examples

Every example is compiled by the EK9 compiler: the first fails exactly as captioned, the others fix it.

Fails with E08097: assign-if-unset ':=?' always applies because the target is always unset here - it is a plain assignment; use ':=' (or declare the variable with the value)

#!ek9
defines module error.e08097.bad

  defines function

    //Nothing in the environment: unset
    fromEnvironment()
      <- rtn as String: String()

    resolveSetting()
      <- setting as String: String()
      setting :=? fromEnvironment()
      setting :=? "default"

Fixed: Use ':=?' only where the value may already be set

#!ek9
defines module error.e08097.good

  defines function

    //Nothing saved: unset
    savedName()
      <- rtn as String: String()

    //From a function call the value may be set, so ':=?' decides.
    nameOrDefault()
      <- rtn as String: savedName()
      rtn :=? "anonymous"

    //Declared with no value: ':=?' is the form that fills it.
    greeting()
      <- rtn as String?
      rtn :=? "Hello"

    //Known to be unset inside 'if not total?', so ':=' gives it the value.
    topUp()
      -> amount as Integer
      <- total as Integer: amount
      if not total?
        total := 0

    //A guard also gates its block, which ':=' would not.
    printIfFound()
      found as String?
      if found :=? savedName()
        Stdout().println(found)

Fixed: Start the fallback chain from its first source

#!ek9
defines module error.e08097.good.fix

  defines function

    //Nothing in the environment: unset
    fromEnvironment()
      <- rtn as String: String()

    resolveSetting()
      <- setting as String: fromEnvironment()
      setting :=? "default"

Questions and answers

These Q&A entries show E08097 and its fix: