3 ms·

One small detail - though COND and IF/ELSE are very closely related, they aren't quite the same thing. Early Smalltalk also started out with COND (a right arrow

by jecel 7y ago

One small detail - though COND and IF/ELSE are very closely related, they aren't quite the same thing. Early Smalltalk also started out with COND (a right arrow which made code look like modern switch/case) and later evolved to #ifTrue:ifFalse: which is still postfix, so feels backwards compared to other languages. Lisp and Scheme got more conventional if/else eventually.

LandR 7y ago
Something I discovered about Clojure's cond recently. It usually looks likethis: (cond (< a b) (println "a < b") (> a b) (println "a > b") :else (println "a = b")) I thought the :else had to be :else, but it only needs to be truthy, so it can be anything that isn't false or nil (which makes sense as you want it to always execute that form if no others match). So this is just the same: (cond (< a b) (println "a < b") (> a b) (println "a > b") :hotdog (println "a = b")) Probably obvious to everyone else but it was a bit of a "duh, of course!" moment for me.
- sparkie 7y ago
  It's conventional in Scheme and Lisp to just use `#t` directly to match anything.
- lispm 7y ago
  In Lisp usually all non-nil values mean true. CL-USER 13 > (let ((a 1) (b 2)) (cond ((> a b) 'foo) (:hello :there))) :THERE In some early Lisp dialects not all objects (other than lists and symbols) would be self-evaluating, so one would have to quote them.
- kazinator 7y ago
  In traditional Lisp and common Lisp, and similar dialects, the symbol t is customarily used as the catch all last case. There is an interesting situation is in the case family of constructs which match an input value against keys, in Common Lisp. Inspired by cond, the t symbol also serves as the fallback in case when the key doesn't match the other cases. So that is to say: (case (expr) (a ...) (b ...) (42 ...) (t ...)) ;; <-- this isn't matching on the t symbol! Common Lisp also supports the symbol otherwise in place of t. But the programmer may also sometimes have the t symbol as a specific key value; or likewise the otherwise symbol. That requirement is handled by putting the key into a list: (case (expr) (a ...) ... ((t) ...)) ;; <- match on t
- wostusername 7y ago
  In Haskell you usually use `otherwise` for the last guard clause, like so: f x | x < 0 = ... | otherwise = ... It took me a while to realize that `otherwise` isn't a keyword or bit of syntax. It's just a variable bound to True: https://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.Base.html#otherwise https://hackage.haskell.org/package/base-4.12.0.0/docs/src/G...
lispm 7y ago
> COND and IF/ELSE are very closely related In earlier Lisp COND was the primitive and IF was a macro expanding into COND. In newer Lisp it's IF which is the primitive and COND which is the macro.

by jecel 7y ago

LandR 7y ago

Something I discovered about Clojure's cond recently. It usually looks likethis: (cond (< a b) (println "a < b") (> a b) (println "a > b") :else (println "a = b")) I thought the :else had to be :else, but it only needs to be truthy, so it can be anything that isn't false or nil (which makes sense as you want it to always execute that form if no others match). So this is just the same: (cond (< a b) (println "a < b") (> a b) (println "a > b") :hotdog (println "a = b")) Probably obvious to everyone else but it was a bit of a "duh, of course!" moment for me.

sparkie 7y ago
It's conventional in Scheme and Lisp to just use `#t` directly to match anything.
lispm 7y ago
In Lisp usually all non-nil values mean true. CL-USER 13 > (let ((a 1) (b 2)) (cond ((> a b) 'foo) (:hello :there))) :THERE In some early Lisp dialects not all objects (other than lists and symbols) would be self-evaluating, so one would have to quote them.
kazinator 7y ago
In traditional Lisp and common Lisp, and similar dialects, the symbol t is customarily used as the catch all last case. There is an interesting situation is in the case family of constructs which match an input value against keys, in Common Lisp. Inspired by cond, the t symbol also serves as the fallback in case when the key doesn't match the other cases. So that is to say: (case (expr) (a ...) (b ...) (42 ...) (t ...)) ;; <-- this isn't matching on the t symbol! Common Lisp also supports the symbol otherwise in place of t. But the programmer may also sometimes have the t symbol as a specific key value; or likewise the otherwise symbol. That requirement is handled by putting the key into a list: (case (expr) (a ...) ... ((t) ...)) ;; <- match on t
wostusername 7y ago
In Haskell you usually use `otherwise` for the last guard clause, like so: f x | x < 0 = ... | otherwise = ... It took me a while to realize that `otherwise` isn't a keyword or bit of syntax. It's just a variable bound to True: https://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.Base.html#otherwise https://hackage.haskell.org/package/base-4.12.0.0/docs/src/G...

lispm 7y ago

> COND and IF/ELSE are very closely related In earlier Lisp COND was the primitive and IF was a macro expanding into COND. In newer Lisp it's IF which is the primitive and COND which is the macro.