CODEWITHSUNDEEP
scala
Tim
Tim

Reputation: 2050

Scala chaining functions with andThen type mismatch

I have a bunch of functions that clean text and splits them into words. Minimal example:

val txt = "Mary had a @little \nlamb"
val stopwords = Seq("a")
def clean(text: String): String = text.replaceAll("\n*\r*", "")
def tokenize(text: String): Seq[String] = text.split("\\s")

val cleaned = clean(txt)
val tokens = tokenize(cleaned)

This code works as expected. However not really idiomatic. I had hoped to do this: 

clean(txt) andThen tokenize

But the compiler complains about this with the error type mismatch; required: Char => ? at the tokenize function.

What am I missing here?

Upvotes: 4

Views: 835

Answers (1)

Yuval Itzchakov
Yuval Itzchakov

Reputation: 149538

clean returns a String. You're trying to use andThen on a String instance (since you're invoking the method with clean(txt)) and the compiler infers it as a PartialFunction[Char, ?] (because WrappedString inherits AbstractSeq[Char] which inherits PartialFunction[Char, A]). That is why you're seeing the type mismatch. If you want to compose the two together, turn them into function types using eta-expansion:

val res = clean _ andThen tokenize
println(res(txt))

Function composition works on Scala functions, not methods (there is a distinction), and that is why we have to first expand the method to a function (clean _), and then the compiler will be able to infer tokenize for us without the need to manually expand it.

Upvotes: 4

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