Variable dependent tasks in SBT

Question

I can make a Makefile that has a target that processes all sources in the directory.

SOURCE_DIR := src

TARGET_DIR := target

SOURCES := $(wildcard $(SOURCE_DIR)/*)

$(TARGET_DIR)/%: $(SOURCE_DIR)/%
    md5sum $^ > $@

all: $(SOURCES:$(SOURCE_DIR)/%=$(TARGET_DIR)/%)

A nice advantage here is that each file is a separate target, so they can be processed incrementally, and concurrently. The concurrent part is important in this situation.

---

I am trying to something similar with SBT, but am finding it surprisingly difficult. The SBT analog of a Make target sees to be a task, so I try creating one task that aggregate a variable number of smaller tasks.

import org.apache.commons.codec.digest.DigestUtils

all <<= Def.task().dependsOn({
    file(sourceDir.value).listFiles.map { source =>
        val target = rebase(sourceDir.value, targetDir.value)(f)
        Def.task {
            IO.write(target, DigestUtils.md5Hex(IO.readBytes(source)))
        }
    }
}: _*)

I get the error

`value` can only be used within a task or setting macro, such as :=, +=, ++=,
Def.task, or Def.setting

How can I make a proper SBT build file that resembles my Makefile, with a dynamic number of concurrent targets/tasks?

Answer 1

I needed flatMap.

all <<= (sourceDir, targetDir).flatMap { (sourceDir, targetDir) =>
    task{}.dependsOn({
        file(sourceDir).listFiles.map { source =>
            task {
                val target = rebase(sourceDir, targetDir)(f)
                IO.write(target, DigestUtils.md5Hex(IO.readBytes(source)))
            }
        }
    }: _*)
}

There might be a slicker way to do task{}.dependsOn(...: _*), but I don't know what it is.