What does func!thing mean?

[ProgrammingGhost]

I'm a D noob. ".map!(a => a.length)" seems like the lambda is 
passed into the  template. ".map!split" just confuses me. What is 
split? I thought only types can be after "!". I would guess split 
is a standard function but then shouldn't it be map!(split)?

     const wordCount = file.byLine()                  // Read lines
                           .map!split                 // Split 
into words
                           .map!(a => a.length)       // Count 
words per line
                           .reduce!((a, b) => a + b); // Total 
word count