[source]

compiler/typecheck/TcArrows.hs

Note [Arrow overview]

[note link]

Here’s a summary of arrows and how they typecheck. First, here’s a cut-down syntax:

expr ::= ....
      |  proc pat cmd

cmd ::= cmd exp                    -- Arrow application
     |  \pat -> cmd                -- Arrow abstraction
     |  (| exp cmd1 ... cmdn |)    -- Arrow form, n>=0
     |  ... -- If, case in the usual way

cmd_type ::= carg_type --> type

carg_type ::= ()
           |  (type, carg_type)
Note that
  • The ‘exp’ in an arrow form can mention only “arrow-local” variables
  • An “arrow-local” variable is bound by an enclosing cmd binding form (eg arrow abstraction)
  • A cmd_type is here written with a funny arrow “–>”, The bit on the left is a carg_type (command argument type) which itself is a nested tuple, finishing with ()
  • The arrow-tail operator (e1 -< e2) means
    (| e1 <<< arr snd |) e2