Type mismatch error when assigning literal to Float

Use the following code:

val f: Float = 3f

If you don't add f suffix, the variable will be treated as double.

It's the same thing, that happens in java, since Double and Float wrap over java's double and float respectively:

jshell> float f = 3.0
|  Error:
|  incompatible types: possible lossy conversion from double to float
|  float f = 3.0;
|            ^-^

jshell> float f = 3.0f
f ==> 3.0

To have a Float you have to suffix it with a f:

scala> val f: Float = 3.0f
f: Float = 3.0

By default a floating number is a Double as the exception suggests.

From floating-point-literals (provided by Mario Galic):

Floating point literals are of type Float when followed by a floating point type suffix F or f, and are of type Double otherwise.

This behavior is fixed in Scala 3 (Dotty):

Starting dotty REPL...
Dotty compiler version 0.22.0-RC1 -- Copyright 2002-2020, LAMP/EPFL
scala> val x: Float = 3.14
val x: Float = 3.14

scala> Seq(3.14, 1)
val res0: Seq[Double] = List(3.14, 1.0)

scala> Seq[Float](3.14, 1)
val res1: Seq[Float] = List(3.14, 1.0)

It does not infer a type for a literal that would incur a lossy conversion:

scala> Seq(3.14f, 2147483646)
val res2: Seq[AnyVal] = List(3.14, 2147483646)

scala> Seq(3.14f, 2147483647)
val res3: Seq[Float] = List(3.14, 2147483647)

There is an open ticket to warn when a lossy conversion is required, but in general it works pretty well.