Mirah Assignment corner cases

notes.md

Multiple Assignment general cases

The fundamental question is whether the return type of an assignment is rhs's type or lhs's.

#  B = C => ?
   b = c

should a = b = c => a = c; b = c or b = c; a = b?

middle type narrower

a = b = c

where

• a is uninferred,
• b & c have been inferred
• b's type is narrower than c's

should a's type be b's or c's?

middle type wider

a = b = c

where

• a is uninferred,
• b & c have been inferred
• b's type is wider than c's

should a's type be b's or c's?

assignment & return cases

middle type narrower

def a
  b = c
end

where

• a's return type is uninferred,
• b & c have been inferred
• b's type is narrower than c's

should a's return type be b's or c's?

middle type wider

def a
  b = c
end

where

• a's return type is uninferred,
• b & c have been inferred
• b's type is wider than c's

should a's return type be b's or c's?

a's return type is wider than b & c

B < ... A C < ... < B ... < A

def a : A
  b = c
end

where

• a's return type is wider than b & cs,
• a & b & c have been inferred
• b's type is wider than c's

fine

a's return type is wider than c, but narrower than b

C < A < B

def a : A
  b = c
end

where

• a's return type is wider than c's,
• a's return type is narrower than c's,
• a & b & c have been inferred
• b's type is wider than c's

should this be an inference error? Or should we assume a returns c?