Editing Mesa (programming language) (section)

==Main features==

===Semantics===
Mesa was a [[strongly typed programming language]] with type-checking across module boundaries, but with enough flexibility in its type system that heap allocators could be written in Mesa.<ref name="geschke">{{cite journal |doi=10.1145/359763.359771 |first1=Charles |last1=Geschke |author-link=Charles Geschke |first2=James H. |last2=Morris |author-link2=James H. Morris |first3=Edwin H. |last3=Satterthwaite |date=August 1977 |title=Early Experience with Mesa |journal=Communications of the ACM |volume=20 |issue=8 |pages=540–552 |s2cid=15737342 |doi-access=free }}</ref>

Due to its strict separation between interface and implementation, Mesa allows true incremental compilation and encourages [[computer architecture|architecture]]- and [[platform (computing)|platform]]-independent programming.  They also simplified source-level [[debugger|debugging]], including remote debugging via the [[Ethernet]].

Mesa had rich [[exception handling]] facilities, with four types of exceptions.  It had support for thread synchronization via monitors. Mesa was the first language to implement monitor BROADCAST, a concept introduced by the Pilot operating system.<ref name="monit">{{cite journal |doi=10.1145/358818.358824 |url=http://research.microsoft.com/lampson/23-ProcessesInMesa/Abstract.html |title=Experience with Processes and Monitors in Mesa |first1=Butler W. |last1=Lampson |author-link=Butler W. Lampson |first2=David D. |last2=Redell  |journal=Communications of the ACM |volume=23 |issue=2 |pages=105–117 |date=February 1980 |citeseerx=10.1.1.142.5765 |s2cid=1594544 }}</ref>

===Syntax===
Mesa has an "imperative" and "algebraic" [[syntax]], based on [[ALGOL (programming language)|ALGOL]] and [[Pascal (programming language)|Pascal]] rather than on BCPL or [[C (programming language)|C]]; for instance, [[block (programming)|compound command]]s are indicated by the {{Mono|BEGIN}} and {{Mono|END}} keywords rather than [[brace (punctuation)|brace]]s. In Mesa, all keywords are written in uppercase.<ref name="manual" />

Due to PARC's using the 1963 variant of [[ASCII]] rather than the more common 1967 variant, the Alto's character set included a left-pointing arrow (←) rather than an underscore. The result of this is that Alto programmers (including those using Mesa, Smalltalk etc.) conventionally used [[camelCase]] for compound identifiers, a practice which was incorporated in PARC's standard programming style. On the other hand, the availability of the left-pointing arrow allowed them to use it for the assignment operator, as it originally had been in ALGOL.

When the Mesa designers wanted to implement an exception facility, they hired a recent M.Sc. graduate{{who|date=October 2021}} from Colorado<!-- John B. Goodenough ? --> who had written his thesis on exception handling facilities in algorithmic languages.  This led to the richest exception facility for its time, with primitives {{Mono|SIGNAL}}, {{Mono|ERROR}}, {{Mono|ABORT}}, {{Mono|RETRY}}, {{Mono|CATCH}}, and {{Mono|CONTINUE}}.  As the language did not have type-safe checks to verify full coverage for signal handling, uncaught exceptions were a common cause of bugs in released software.