Why won't the swift compiler accept the following map closure

Question

Why doesn't the swift compiler accept the following map closure

let coords = ["5.691663622856139,45.11744852318003","5.691341757774353,45.11710783320859"]
let nodes = coords.map({
   let latlon = $0.componentsSeparatedByString(",")
   let lon = Double(latlon[0])!
   let lat = Double(latlon[1])!
   return CLLocation(latitude: lat, longitude: lon)
})

cannot invoke map with an argument list of type (@noescape (String) throws -> _)

and yet

let nodes:[CLLocation] = coords.map({
   let latlon = $0.componentsSeparatedByString(",")
   let lon = Double(latlon[0])!
   let lat = Double(latlon[1])!
   return CLLocation(latitude: lat, longitude: lon)
})

works well.

I think @Anton has got it right. Swift's type inference in closures can be a bit silly sometimes. — Will Richardson, Jan 01 '16 at 21:59
The closure return type is inferred automatically *only* for single-expression closures, or from the context. Compare e.g. http://stackoverflow.com/a/34115788/1187415 for a similar issue. — Martin R, Jan 01 '16 at 22:01
Man, your knowledge on Swift is impressive. thanks for sharing it ! — Matthieu Riegler, Jan 01 '16 at 22:33

score 3 · Accepted Answer · answered Jan 01 '16 at 21:57

Swift compiler (still) gets confused when it comes to type-inference for "complex" closures with shorthand arguments, and needs some help along the following lines:

let coords = ["5.691663622856139,45.11744852318003","5.691341757774353,45.11710783320859"]
let nodes = coords.map() {
    (stringCoordinates: String) -> CLLocation in // <- Just specify input/output

    let latlon = stringCoordinates.componentsSeparatedByString(",")
    let lon = Double(latlon[0])!
    let lat = Double(latlon[1])!
    return CLLocation(latitude: lat, longitude: lon)
}

Why won't the swift compiler accept the following map closure

1 Answers1