First-class binary operations

[rolyp]

Given more regular treatment of primitives in #138 and #128, it would be easy now to allow first-class binary operations, perhaps using Haskell-style “section” syntax.

Doing this as part of the PureScript port and in particular #301. Will involve porting more of the TypeScript implementation, in particular the bits that deal with binary operations.

• full Haskell-style section syntax – see comment below
• BinaryOp datatype, binaryOps map from strings to operators
• BinaryApp expression form
• basic prettyprinting for BinaryApp
• delete Typed?
• BinaryApp trace form
• delete Add form
• Op expression and value forms
• evaluation and forward-slicing rules for Op expressions
• (op) to denote first-class operator
• App to handle first-class operators as well as closures
• forward-slicing rules for App/operator cases
• Op form should take an operator name; eval rule should look this up in the environment
• populate initial environment with an entry for each primitive

[rolyp]

Full Haskell-style section syntax, e.g. (3 * 4 +), would involve a non-trivial modification buildExprParser, which currently only supports trees of binary operations. Useful low-hanging fruit would be to allow (+) to denote the operator, which would allow putting expressions like (+) 5 and flip (+) 5 in the function position of an application.