Migrating off deprecated syntax

Deprecated syntax keeps working. It always leaves in three stages: deprecated, migrated, removed - never straight to removed.

ecko fix --migrate --check src/    # report what needs it, write nothing
ecko fix --migrate src/            # rewrite in place

The three stages

1. Deprecated. The old form still parses and runs. ecko check warns on it, and ecko fmt rewrites it to the current form as part of an ordinary format pass. 2. Migrate. Run ecko fix --migrate (or just ecko fmt) before the version that removes the old form ships. Both do the same rewrite; --migrate touches only the deprecated constructs, so the diff is the migration and nothing else - useful on a project that is not kept formatted with fmt. 3. Removed. A later version deletes parser support for the old form. Unmigrated code stops parsing at that point, not before - there is no silent behaviour change in between.

Today's deprecations

const to let

const limit = 100   # deprecated: identical to `let`
print(limit)

const was an early alias for let. There is no behavioural difference to reason about - the migration is a rename:

let limit = 100
print(limit)

Pipe lambdas to fn(params) body

double = |x| x * 2   # deprecated pipe-lambda form
print(double(21))

fn(params) body is the canonical anonymous form and is what ecko fmt produces from the pipe form:

double = fn(x) x * 2
print(double(21))

See Functions & lambdas for the full lambda syntax.

One-time codemods are different from --migrate

Two rewrites belong to specific past breaking releases and are not run by plain --migrate, because they are reinterpretations rather than canonicalisations - running one twice on already-migrated code would corrupt it (see ecko fix for why):

ecko fix --migrate --only=bytes src/     # *_decode -> *_decode_text, net.recv -> recv_text
ecko fix --migrate --only=exports src/   # the old `export { ... }` block to inline `export`

Run each once, on code you know has not already been through it.

Checking without changing anything

ecko fix --migrate --check src/ reports which files need a rewrite and exits non-zero if any do, without touching a byte - the shape you want in CI, ahead of the version that removes the old form:

ecko fix --migrate --check src/

See ecko check for the warnings that surface deprecated syntax during ordinary development, and ecko fmt for the formatter that rewrites it as a side effect of formatting.