Optional bar before first match clause
From Successor ML
Contents |
Introduction
We propose a minor syntax extension to allow an optional bar before the first clause in a match.
Motivation
SML syntax separates match clauses with a bar "|". The usual coding convention is to lay out matches such that the bar comes before each clause. However, the first clause is an unpleasent special case:
case exp0 of
pat1 => exp1
| pat2 => exp2
| pat3 => exp3
Taking aesthethic considerations aside, the assymmetry between the cases is a nuisance for editing, because clauses cannot be reordered by a simple cut & paste operation. We hence propose allowing an additional bar to optionally appear before the first clause, such that the above can be written as:
case exp0 of
| pat1 => exp1
| pat2 => exp2
| pat3 => exp3
For consistency, we make the same extension for function value bindings, and for datatype declarations. For instance,
datatype 'a exp =
| Const of 'a
| Var of string
| Lambda of string * 'a exp
| App of 'a exp * 'a exp
A similar extension has been made in Objective Caml.
Assumptions
None.
Syntax
Defined by the following modifications to the Definition:
- In Section 2.8, Figure 4, and Appendix B, Figure 20, change the productions for exception handling, functions, and case expressions (only in the latter figure) to, respectively:
[exp ::=] exp "handle" <"|"> match handle exception
"fn" <"|"> match function
"case" exp "of" <"|"> match case analysis
- In Section 2.8, Figure 4, and Appendix B, Figure 21, change the production for datatype bindings to:
datbind ::= tyvarseq tycon "=" <"|"> conbind <"and" datbind>
- In Section 3.4, Figure 7, change the productions for datatype descriptions to:
datdesc ::= tyvarseq tycon "=" <"|"> condesc <"and" datdesc>
- In Appendix A, Figure 15, change the rule for case expressions to
+-----------------------+---------------------+ | case exp of <|> match | (fn <|> match)(exp) | +-----------------------+---------------------+
- In Figure 17, change the first line in the definition of function clauses to
+------------------------------------------------+-------------------------------------------+ | | [...] | | <|> <op>vid atpat11 ... atpat1n <: ty1> = exp1 | <|> (atpat11,...,atpat1n) => exp1 <: ty1> | | [...] | [...] | +------------------------------------------------+-------------------------------------------+
- In Appendix B, Figure 21, change the production for fvalbind accordingly.
Static Semantics
Defined by the following modifications to the Definition:
- In Section 4.10, Expressions, modify the syntax in the conclusion of rules 10 and 12 accordingly.
Dynamic Semantics
Defined by the following modifications to the Definition:
- In Section 6.7, Expressions, modify the syntax in the conclusion of rules 104-106 and 108 accordingly.
Interactions
None.
Compatibility
This is a conservative change.
Implementation
Trivial.
