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What I wish I knew when learning OCaml (2018)

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Re: What I wish I knew when learning OCaml (2018)

#31

Post by skissane »

kragen wrote: post #17
WastingMyTime89 wrote: post #9

Earlier quoted context omitted.

> I annotated the hell out of my programs, and completely avoided OCaml's type inference as much as I could because I saw that it could not guess what I meant. The type checker and the type inference parts of the compiler are one and the same. If the type checker can’t infer what you mean you are most likely writing invalid code. > Error messages like "This expression is of type X but an expression was expected of ty…

> Ocaml is a strongly typed static language. As such your code has to respect type constraints. It’s very much a feature not a bug. That's reasonable if the error message were "This expression is of type X but an expression was expected of type Y ". But the worst error message in the OCaml interactive toplevel is when they're the same. This happened when you defined a new type named X (which, incidentally, must be lo…

You can get the same thing in Java essentially: two classes with the same name but loaded by different class loaders are incompatible. You’d get errors at runtime that you can’t cast X to X. I think they’ve made the exception messages more helpful in more recent Java versions, but in older versions the errors were very confusing to newbies because they didn’t mention the class loader difference which was the actual cause of them

Re: What I wish I knew when learning OCaml (2018)

#32

Post by cassepipe »

maweki wrote: post #30
cassepipe wrote: post #27

Could someone please explain ? : type 'a list = 'a :: 'a list | [] The article says "::" is a Data Constructor. I can make sense of type 'a = Left of 'a | Right of 'a where Right and Left are the Data constructors but I don't see the link with the part I don't understand.

A list is either the empty list [] or Two pieces of data (with the constructor ::) with 1 being the head of the list (type 'a) and one being the tail of the list (type 'a list, a recursive definition). :: is an allowed identifier in oCaml. If that is not allowed, the typical names are Cons for :: and Nil for []

Thanks. I had not understood it could be recursive.

Do you know Cons' meaning ?

Also why does :: act as a separator... ? Is that something you ca do with any Data constructor, I mean can I write type 'a = 'a Cons of 'a | Nil of 'a ?

Re: What I wish I knew when learning OCaml (2018)

#33

Post by marginalia_nu »

nnoitra wrote: post #4
pharmakom wrote: post #2

A good list. ML languages push you (kicking and screaming) into the pit of success.

They're barely used in industry.

Even if functional programming itself isn't used, a lot of its patterns are extremely useful even outside of functional programming.

Code returning side effects rather than having side effects, for example, is great, and I find myself returning to the principle in a lot of the stuff I design as an architectural principle.

Re: What I wish I knew when learning OCaml (2018)

#34

Post by eatonphil »

dgan wrote: post #5

TIL that Ocaml/SML are compiled into lambda expressions... I literally thought lambda calculus is only used in CS classes. OCaml's syntax is pretty annoying but type inference is actually amazing.. I changed my mind over it, as previously I thought explicit type annotations are simpler. Turns out, it would be humanly impossible to explicitly annotate every piece of OCaml, just let the compiler do it for you

Did you learn that from some other article? I couldn't find "lambda" in this page.

I think it's too much to say that all Standard ML compilers work or one way or another. There are 6 major compilers and a number of minor ones.

They don't really follow the same approaches in general.

Re: What I wish I knew when learning OCaml (2018)

#36

Post by willmorrison »

cassepipe wrote: post #32
maweki wrote: post #30

Earlier quoted context omitted.

A list is either the empty list [] or Two pieces of data (with the constructor ::) with 1 being the head of the list (type 'a) and one being the tail of the list (type 'a list, a recursive definition). :: is an allowed identifier in oCaml. If that is not allowed, the typical names are Cons for :: and Nil for []

Thanks. I had not understood it could be recursive. Do you know Cons' meaning ? Also why does :: act as a separator... ? Is that something you ca do with any Data constructor, I mean can I write type 'a = 'a Cons of 'a | Nil of 'a ?

Here is the Wikipedia page: https://en.m.wikipedia.org/wiki/Cons

Re: What I wish I knew when learning OCaml (2018)

#37

Post by gpderetta »

cassepipe wrote: post #32
maweki wrote: post #30

Earlier quoted context omitted.

A list is either the empty list [] or Two pieces of data (with the constructor ::) with 1 being the head of the list (type 'a) and one being the tail of the list (type 'a list, a recursive definition). :: is an allowed identifier in oCaml. If that is not allowed, the typical names are Cons for :: and Nil for []

Thanks. I had not understood it could be recursive. Do you know Cons' meaning ? Also why does :: act as a separator... ? Is that something you ca do with any Data constructor, I mean can I write type 'a = 'a Cons of 'a | Nil of 'a ?

Cons comes from Lisp: https://en.wikipedia.org/wiki/Cons . In practice a cons cell is a list node represented as a pair of pointers, one pointing to the contained element, the other to the next node (or null, or another element for degenerate lists).

Re: What I wish I knew when learning OCaml (2018)

#38

Post by kragen »

skissane wrote: post #31
kragen wrote: post #17

Earlier quoted context omitted.

> Ocaml is a strongly typed static language. As such your code has to respect type constraints. It’s very much a feature not a bug. That's reasonable if the error message were "This expression is of type X but an expression was expected of type Y ". But the worst error message in the OCaml interactive toplevel is when they're the same. This happened when you defined a new type named X (which, incidentally, must be lo…

You can get the same thing in Java essentially: two classes with the same name but loaded by different class loaders are incompatible. You’d get errors at runtime that you can’t cast X to X. I think they’ve made the exception messages more helpful in more recent Java versions, but in older versions the errors were very confusing to newbies because they didn’t mention the class loader difference which was the actual c…

In OCaml I think this could only happen with the interpreter (e.g., an interactive toplevel). At least today the compiler complains if you try to define two types with the same name in the same context.

The problem is that it would happen often in the interpreter when you were interactively trying stuff out, because when you're trying stuff out, you change the definitions of things.

Python actually sort of has the same problem not only when you use an interactive interpreter but even when you reload a module: the new class definitions don't modify the old one, they just get bound to the same name. So it's easy to end up with two alglayout.Vbox classes or two diff.Formula classes in the same interactive interpreter at the same time. But it's much less of a problem in Python because Python usually doesn't care what class things are, just what methods they define, so objects belonging to both classes can coexist peacefully. The usual exception is when you have an isinstance check somewhere.

Re: What I wish I knew when learning OCaml (2018)

#39

Post by frou_dh »

eatonphil wrote: post #34
dgan wrote: post #5

TIL that Ocaml/SML are compiled into lambda expressions... I literally thought lambda calculus is only used in CS classes. OCaml's syntax is pretty annoying but type inference is actually amazing.. I changed my mind over it, as previously I thought explicit type annotations are simpler. Turns out, it would be humanly impossible to explicitly annotate every piece of OCaml, just let the compiler do it for you

Did you learn that from some other article? I couldn't find "lambda" in this page. I think it's too much to say that all Standard ML compilers work or one way or another. There are 6 major compilers and a number of minor ones. They don't really follow the same approaches in general.

At a higher level that that, it's cool that even in SML's already sparse surface-level syntax, quite of a bit of it is just sugar for each other: https://i.imgur.com/pkSg4xm.png

Re: What I wish I knew when learning OCaml (2018)

#40

Post by bmc7505 »

ML really is a beautiful and under-appreciated family of languages. Haskell is a bit too supernatural for my taste, but ML and its derivatives hit the sweet spot between powerful type systems and natural syntax. As a Kotlin developer, I find reading OCaml code a breeze and the compiler is refreshingly snappy (much faster than Gradle). I just wish it had better developer tools.
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