How to compose two functions? - scala

Question

I have two functions, that I am trying to compose it:
private def convert(value: String)
: Path = decode[Path](value) match {
private def verify(parsed: Path)
: Path = parsed.os match {
I've tried as following:
verify compose convert _
The compiler complains:
[error] Unapplied methods are only converted to functions when a function type is expected.
[error] You can make this conversion explicit by writing `verify _` or `verify(_)` instead of `verify`.
[error] verify compose convert _
[error] ^
[error] one error found
I am trying to accomplish the following:
def process(value: String)
: Path =
verify(convert(value))
What am I doing wrong?

Answer 1

The problem is not everything in scala is a function. convert and verify in your example are methods not functions.
1) If you want to compose functions, define as functions as example below,
functions,
scala> def convert: String => String = (value: String) => "converted"
convert: String => String
scala> def verify = (value: String) => if(value == "converted") "valid" else "invalid"
verify: String => String
compose fns
scala> def vc = verify compose convert
vc: String => String
apply fn composition to functor
scala> Some("my-data").map{ vc.apply }
res29: Option[String] = Some(valid)
2) The another way is convert your existing methods to functions as below,
scala> def vc = (verify _ compose (convert _))
vc: String => String
scala> Some("data to convert").map { vc.apply }
res36: Option[String] = Some(valid)
References
https://twitter.github.io/scala_school/pattern-matching-and-functional-composition.html
Chapter 21 - Functions/Function literals
Function composition of methods, functions, and partially applied functions in Scala