4 ms·

'In Haskell, everything is a function' is absolutely wrong. People who don't know much about Haskell often say this. There's a post by Conal Elliott about this

by tumult 15y ago

'In Haskell, everything is a function' is absolutely wrong. People who don't know much about Haskell often say this. There's a post by Conal Elliott about this incorrect meme: http://conal.net/blog/posts/everything-is-a-function-in-haskell http://conal.net/blog/posts/everything-is-a-function-in-hask... It's simply not true. In fact, because Haskell is statically typed, it's very easy to tell what's a function and what's not: If its type doesn't have an -> in it, it's not a function.

noblethrasher 15y ago
... I know that 3 is not literally function in the Haskell type system but it's an easy way to explain it to people less familiar with the language (not that I assume freyrs3 is one of those). Also, while it might be true that "people who don't know much about Haskell" often say that everything is a function, it's also the case that people who do know a lot about the language find the notion to be pedagogically useful: On page 13 of The Haskell School of Expression, Paul Hudak offers that pi (which is of type Floating a => a and thus clearly not a function) "can be thought of as a function with no arguments".
- alipang 15y ago
  Also, the definition of an infinite list of ones ones = 1 : ones is a non-function value, but is recursively applied to itself. Might help illustrate why this is a useful notion in Haskell.
  - lurker14 15y ago
    To be precise. "ones" is not "recursively applied to itself". "ones" is not a function. "(:)" is a function. In actuality, "ones" is both an argument and a result of the (:) function (aka "cons"). This works in Haskell because "ones" is not an atomic value or function; "ones" is a "lazy" structure, where some of the elements's values depend on other element's values, and Haskell uses a "call-by-need" graph reduction algorithm to evaluate arbitrarily complex* computations down to values, not in any sort of sraihtforward order suggested by "argument -> function -> result -> lvalue" in an imperative or strict language. Laziness is at the core of the runtime system's model for computing a result from a program, and it's not something you can translate line-by-line from a non-lazy program, even a functional language program. I'm not just being pedantic. Haskell (with a runtime like GHC, required for this conversation to really make sense) really is different from other programming systems. ("Programming system"! Not just "language"!). The runtime system ("RTS") is not just a C runtime that provides a statement language and shim over OS system calls. The RTS is not "just" a VM like Java VM. The RTS is (metaphorically) like perl's "perl -nple" or a SQL RDBMS query planner, where the runtime is itself a program that takes your code as input and does its own extremely sophisticated computation that is not at all visible in your program's code. [] Arbitrarily* complex, but still "causal" structures that admit at least one valid order of evaluation. Trying to define "ones = ones" or other non-disentanglable cyclic dependencies leads to a runtime failure ("<loop>" exception). [] In some cases, the graph solver might practically fail to solve a graph that is theoretically solvable. Let's ignore that.
    - gtani 15y ago
      That's good info, GHC is an pretty amazing feat of engineering, there are some good summaries of STG, C--, etc for further reading: http://darcs.haskell.org/ghc/docs/comm/the-beast/stg.html http://darcs.haskell.org/ghc/docs/comm/the-beast/stg.html http://hackage.haskell.org/trac/ghc/wiki/Commentary/Compiler/StgSynType http://hackage.haskell.org/trac/ghc/wiki/Commentary/Compiler... http://www.reddit.com/r/programming/comments/i4jb1/haskells_evaluation_isnt_magic/c21369j http://www.reddit.com/r/programming/comments/i4jb1/haskells_... ------------- Also, Simon M started Google +ing about what he's up to https://plus.google.com/u/0/107890464054636586545/posts https://plus.google.com/u/0/107890464054636586545/posts

by tumult 15y ago

noblethrasher 15y ago

... I know that 3 is not literally function in the Haskell type system but it's an easy way to explain it to people less familiar with the language (not that I assume freyrs3 is one of those). Also, while it might be true that "people who don't know much about Haskell" often say that everything is a function, it's also the case that people who do know a lot about the language find the notion to be pedagogically useful: On page 13 of The Haskell School of Expression, Paul Hudak offers that pi (which is of type Floating a => a and thus clearly not a function) "can be thought of as a function with no arguments".

alipang 15y ago
Also, the definition of an infinite list of ones ones = 1 : ones is a non-function value, but is recursively applied to itself. Might help illustrate why this is a useful notion in Haskell.
- lurker14 15y ago
  To be precise. "ones" is not "recursively applied to itself". "ones" is not a function. "(:)" is a function. In actuality, "ones" is both an argument and a result of the (:) function (aka "cons"). This works in Haskell because "ones" is not an atomic value or function; "ones" is a "lazy" structure, where some of the elements's values depend on other element's values, and Haskell uses a "call-by-need" graph reduction algorithm to evaluate arbitrarily complex* computations down to values, not in any sort of sraihtforward order suggested by "argument -> function -> result -> lvalue" in an imperative or strict language. Laziness is at the core of the runtime system's model for computing a result from a program, and it's not something you can translate line-by-line from a non-lazy program, even a functional language program. I'm not just being pedantic. Haskell (with a runtime like GHC, required for this conversation to really make sense) really is different from other programming systems. ("Programming system"! Not just "language"!). The runtime system ("RTS") is not just a C runtime that provides a statement language and shim over OS system calls. The RTS is not "just" a VM like Java VM. The RTS is (metaphorically) like perl's "perl -nple" or a SQL RDBMS query planner, where the runtime is itself a program that takes your code as input and does its own extremely sophisticated computation that is not at all visible in your program's code. [] Arbitrarily* complex, but still "causal" structures that admit at least one valid order of evaluation. Trying to define "ones = ones" or other non-disentanglable cyclic dependencies leads to a runtime failure ("<loop>" exception). [] In some cases, the graph solver might practically fail to solve a graph that is theoretically solvable. Let's ignore that.
  - gtani 15y ago
    That's good info, GHC is an pretty amazing feat of engineering, there are some good summaries of STG, C--, etc for further reading: http://darcs.haskell.org/ghc/docs/comm/the-beast/stg.html http://darcs.haskell.org/ghc/docs/comm/the-beast/stg.html http://hackage.haskell.org/trac/ghc/wiki/Commentary/Compiler/StgSynType http://hackage.haskell.org/trac/ghc/wiki/Commentary/Compiler... http://www.reddit.com/r/programming/comments/i4jb1/haskells_evaluation_isnt_magic/c21369j http://www.reddit.com/r/programming/comments/i4jb1/haskells_... ------------- Also, Simon M started Google +ing about what he's up to https://plus.google.com/u/0/107890464054636586545/posts https://plus.google.com/u/0/107890464054636586545/posts