In OCaml, I obtain an error I do not understand when passing arguments which should work to Printf.printf. It is probably because I do not understand that function completely, but I cannot pin down what does not work.
First, I define a function (used for logging):
utop # let log verbosity level str =
if level <= verbosity then (
Printf.printf "\nLevel %i: " level;
Printf.printf str);;
val log : int -> int -> (unit, out_channel, unit) format -> unit = <fun>
All seems well, but then I obtain this:
utop # log 0 0 "%i" 0;;
Error: This function has type
int -> int -> (unit, out_channel, unit) format -> unit
It is applied to too many arguments; maybe you forgot a `;'.
although the following works:
utop # Printf.printf;;
- : ('a, out_channel, unit) format -> 'a = <fun>
utop # Printf.printf "%i" 0;;
0- : unit = ()
So, how can I define a function which does what log intends to do ?
Edit: Indeed, log 0 0 "%i" 0;; looks like too many arguments (4 instead of 3), but so does Printf.printf "%i" 0;; (2 instead of 1), and it works. With partial application, this gives this:
utop # Printf.printf "%i";;
- : int -> unit = <fun>
utop # log 0 0 "%i";;
Error: This expression has type (unit, unit) CamlinternalFormatBasics.precision
but an expression was expected of type
(unit, int -> 'a) CamlinternalFormatBasics.precision
Type unit is not compatible with type int -> 'a
The printf-like functions are variadic, in the sense that they accept a variable number of arguments. It is not really specific to printf and the family, you can define your own variadic functions in OCaml, this is fully supported by the type system. The only magic of printf is that the compiler translates a string literal, e.g., "foo %d to a value of type format.
Now, let's look at the type of the printf function,
('a, out_channel, unit) format -> 'a
Notice that it returns 'a which is a type variable. Since 'a could be anything it could be also a function. The ('a, out_channel, unit) format is the type of the format string that defines the type of function that is generated by this format string. It is important to understand though, that despite that "foo %d" looks like a string, in fact, it is a special built-in value of type _ format, which has a literal that looks like a string (though not all valid strings are valid literals for the _ format type.
Just to demonstrate that the first argument of printf is not a string, let's try the following,
# Printf.printf ("foo " ^ "%d");;
Line 1, characters 14-29:
1 | Printf.printf ("foo " ^ "%d");;
^^^^^^^^^^^^^^^
Error: This expression has type string but an expression was expected of type
('a, out_channel, unit) format
Now, when we know that printf is not a typical function, let's define a printf-like function ourselves. For that we need to use the kprintf-family of functions, e.g.,
# #show Printf.ksprintf;;
val ksprintf : (string -> 'd) -> ('a, unit, string, 'd) format4 -> 'a
This function takes the function which receives the resulting string which we can log, for example,
# let log fmt = Printf.ksprintf (fun s -> print_endline ("log> "^s)) fmt;;
val log : ('a, unit, string, unit) format4 -> 'a = <fun>
# log "foo";;
log> foo
- : unit = ()
This resulting function looks more like sprintf, i.e., it will play nicely with pretty-printing function that work with string as their output devices (this is a different topic). You may find it more easier to define your logging functions, using either Printf.kfprintf or, much better, using Format.kasprintf or Format.kfprintf. The latter two functions have the following types,
val kasprintf : (string -> 'a) -> ('b, formatter, unit, 'a) format4 -> 'b
val kfprintf : (formatter -> 'a) -> formatter ->
('b, formatter, unit, 'a) format4 -> 'b
But the type of format works with the formatter type (which is an abstraction of the output device) that is the type that the pretty printers (conventionally named pp) are accepting. So the log function defined using the Format module will play better with the existing libraries.
So, using Format.kasprintf we can define your log function as,
# let log verbosity level =
Format.kasprintf (fun msg ->
if level <= verbosity then
Format.printf "Level %d: %s#\n%!" level msg);;
val log : int -> int -> ('a, Format.formatter, unit, unit) format4 -> 'a = <fun>
And here is how it could be used,
# log 0 0 "Hello, %s, %d times" "world" 3;;
Level 0: Hello, world, 3 times
- : unit = ()
Related
So what I want to do is to convert a string into an int and do some error catching on it. I would also like to know where I would put what I want it to do after it fails if it does.
I know how to convert, but I am not sure how to catch it and where the code will jump to after the error
I believe the method for converting it Int.fromString(x)
Thank you.
SML has two approaches to error handling. One, based on raise to raise errors and handle to catch the error, is somewhat similar to how error handling works in languages like Python or Java. It is effective, but the resulting code tends to lose some of its functional flavor. The other method is based on the notion of options. Since the return type of Int.fromString is
string -> int option
it makes the most sense to use the option-based approach.
An int option is either SOME n, where n is and integer, or it is NONE. The function Int.fromString returns the latter if it fails in its attempt to convert the string to an integer. The function which calls Int.fromString can explicitly test for NONE and use the valOf to extract the value in the case that the return value is of the form SOME n. Alternatively, and somewhat more idiomatically, you can use pattern matching in a case expression. Here is a toy example:
fun squareString s =
case Int.fromString(s) of
SOME n => Int.toString (n * n) |
NONE => s ^ " isn't an integer";
This function has type string -> string. Typical output:
- squareString "4";
val it = "16" : string
- squareString "Bob";
val it = "Bob isn't an integer" : string
Note that the clause which starts NONE => is basically an error handler. If the function that you are defining isn't able to handle such errors, it could pass the buck. For example:
fun squareString s =
case Int.fromString(s) of
SOME n => SOME (Int.toString (n * n))|
NONE => NONE;
This has type string -> string option with output now looking like:
- squareString "4";
val it = SOME "16" : string option
- squareString "Bob";
val it = NONE : string option
This would make it the responsibility of the caller to figure out what to do with the option.
The approach to error handling that John explains is elaborated in the StackOverflow question 'Unpacking' the data in an SML DataType without a case statement. The use-case there is a bit different, since it also involves syntax trees, but the same convenience applies for smaller cases:
fun squareString s = Int.fromString s >>= (fn i => SOME (i*i))
Assuming you defined the >>= operator as:
infix 3 >>=
fun NONE >>= _ = NONE
| (SOME a) >>= f = f a
The drawback of using 'a option for error handling is that you have to take into account, every single time you use a function that has this return type, whether it errored. This is not unreasonable. It's like mandatory null-checking. But it comes at the cost of not being able to easily compose your functions (using e.g. the o operator) and a lot of nested case-ofs:
fun inputSqrt s =
case TextIO.inputLine TextIO.stdIn of
NONE => NONE
| SOME s => case Real.fromString s of
NONE => NONE
| SOME x => SOME (Math.sqrt x) handle Domain => NONE
A workaround is that you can build this constant error handling into your function composition operator, as long as all your functions share the same way of expressing errors, e.g. using 'a option:
fun safeSqrt x = SOME (Math.sqrt x) handle Domain => NONE
fun inputSqrt () =
TextIO.inputLine TextIO.stdIn >>=
(fn s => Real.fromString s >>=
(fn x => safeSqrt x))
Or even shorter by applying Eta conversion:
fun inputSqrt () = TextIO.inputLine TextIO.stdIn >>= Real.fromString >>= safeSqrt
This function could fail either because of a lack of input, or because the input didn't convert to a real, or because it was negative. Naturally, this error handling isn't smart enough to say what the error was, so you might want to extend your functions from using an 'a option to using an ('a, 'b) either:
datatype ('a, 'b) either = Left of 'a | Right of 'b
infix 3 >>=
fun (Left msg) >>= _ = Left msg
| (Right a) >>= f = f a
fun try (SOME x) _ = Right x
| try NONE msg = Left msg
fun inputLine () =
try (TextIO.inputLine TextIO.stdIn) "Could not read from stdIn."
fun realFromString s =
try (Real.fromString s) "Could not derive real from string."
fun safeSqrt x =
try (SOME (Math.sqrt x) handle Domain => NONE) "Square root of negative number"
fun inputSqrt () =
inputLine () >>= realFromString >>= safeSqrt
And trying this out:
- inputSqrt ();
9
> val it = Right 3.0 : (string, real) either
- inputSqrt ();
~42
> val it = Left "Square root of negative number" : (string, real) either
- inputSqrt ();
Hello
> val it = Left "Could not derive real from string." : (string, real) either
- (TextIO.closeIn TextIO.stdIn; inputSqrt ());
> val it = Left "Could not read from stdIn." : (string, real) either
(Disclaimer: I am fairly certain that this is not idiomatic in any way. If there is some alternative tree-traversal idiom in OCaml, I'm all ears :) )
I am writing a toy compiler in OCaml, and I would like to have a visitor for my large syntax tree type. I wrote one using classes, but I thought it would be fun to try and implement one using modules/functors. My type hierarchy is massive, so let me illustrate what I'm trying to do.
Consider the following type definitions (making these up on the spot):
type expr =
SNum of int
| SVarRef of string
| SAdd of expr * expr
| SDo of stmt list
and stmt =
SIf of expr * expr * expr
| SAssign of string * expr
Let me briefly illustrate usage. Say, for example, I wanted to collect all of the SVarRefs inside of the program. If I had a mapping visitor (one which visits every node of the tree and does nothing by default), I could do the following (in a perfect world):
module VarCollector : (sig
include AST_VISITOR
val var_refs : (string list) ref
end) = struct
include MapVisitor
let var_refs = ref []
let s_var_ref (s : string) =
var_refs := s::!var_refs
SVarRef(s)
end
(* Where my_prog is a stmt type *)
let refs = begin
let _ = VarCollector.visit_stmt my_prog in
VarCollector.!var_refs
end
I should note that the advantage of having functions for each specific variant is that my actual codebase has a type which both has a large amount of variants and does not make sense to fragment. Variant-specific functions allow the avoiding of repeated iteration implementations for the other variants of a type.
Seems simple enough, but here's the catch: there are different types of visitors. MapVisitor returns the original syntax tree, so it has type
sig
(** Dispatches to variant implementations *)
val visit_stmt : stmt -> stmt
val visit_expr : expr -> expr
(** Variant implementations *)
val s_num : int -> expr
val s_var_ref : string -> expr
val s_add : (expr * expr) -> expr
val s_do : stmt list -> expr
val s_if : (expr * expr * expr) -> stmt
val s_assign : (string * expr) -> stmt
end
At the same time, one might imagine a folding visitor in which the return type is some t for every function. Attempting to abstract this as much as possible, here is my attempt:
module type AST_DISPATCHER = sig
type expr_ret
type stmt_ret
val visit_expr : expr -> expr_ret
val visit_stmt : stmt -> stmt_ret
end
(** Concrete type designation goes in AST_VISITOR_IMPL *)
module type AST_VISITOR_IMPL = sig
type expr_ret
type stmt_ret
val s_num : int -> expr_ret
val s_var_ref : string -> expr_ret
val s_add : (expr * expr) -> expr_ret
val s_do : stmt list -> expr_ret
val s_if : (expr * expr * expr) -> stmt_ret
val s_assign : (string * expr) -> stmt_ret
end
module type AST_VISITOR = sig
include AST_VISITOR_IMPL
include AST_DISPATCHER with type expr_ret := expr_ret
and type stmt_ret := stmt_ret
end
(** Dispatcher Implementation *)
module AstDispatcherF(IM : AST_VISITOR_IMPL) : AST_DISPATCHER = struct
type expr_ret = IM.expr_ret
type stmt_ret = IM.stmt_ret
let visit_expr = function
| SNum(i) -> IM.s_num i
| SVarRef(s) -> IM.s_var_ref s
| SAdd(l,r) -> IM.s_add (l,r)
| SDo(sl) -> IM.s_do sl
let visit_stmt = function
| SIf(c,t,f) -> IM.s_if (c,t,f)
| SAssign(s,e) -> IM.s_assign (s,e)
end
module rec MapVisitor : AST_VISITOR = struct
type expr_ret = expr
type stmt_ret = stmt
module D : (AST_DISPATCHER with type expr_ret := expr_ret
and type stmt_ret := stmt_ret)
= AstDispatcherF(MapVisitor)
let visit_expr = D.visit_expr
let visit_stmt = D.visit_stmt
let s_num i = SNum i
let s_var_ref s = SVarRef s
let s_add (l,r) = SAdd(D.visit_expr l, D.visit_expr r)
let s_do sl = SDo(List.map D.visit_stmt sl)
let s_if (c,t,f) = SIf(D.visit_expr c, D.visit_expr t, D.visit_expr f)
let s_assign (s,e) = SAssign(s, D.visit_expr e)
end
Running this gives me the following error message, however:
Error: Signature Mismatch:
Values do not match:
val visit_expr : expr -> expr_ret
is not included in
val visit_expr : expr -> expr_ret
I know this means that I am not expressing the relationship between the types correctly, but I cannot figure out what the fix is in this case.
Disclaimer: Modules are just records of values accompanied with type definitions. Since there are no types in your modules there is no need to use them at all, just use plain old record types, and you will get one of the idiomatic AST traversal pattern. Soon, you will find out that you need an open recursion, and will switch to class-based approach. Anyway, this was the main reason, why classes were added to OCaml. You may also wrap your classes into a state monad, to fold AST with arbitrary user data.
What concerning your error message, then it is simple, you hide your types with signatures, a common mistake. The easiest solution is to omit an annotation of the return type of a functor at all, or to propagate type equality with a with type expr = expr annotations.
If you need examples of more idiomatic approaches, then for records you can go for ppx mappers, here is an example of different visitors implemented with classes, including those that are wrapped into a state monad.
Is it possible to see the interface of a loaded module in interactive OCaml? I have (unsuccessfully) tried searching for such a possibility, and online docs/sources are not what I am looking for.
The standard trick for this is to define a synonym for the module, which induces the toplevel to list the interface.
$ ocaml
OCaml version 4.00.1
# #load "str.cma";;
# module S = Str;;
module S :
sig
type regexp = Str.regexp
val regexp : string -> regexp
val regexp_case_fold : string -> regexp
val quote : string -> string
val regexp_string : string -> regexp
val regexp_string_case_fold : string -> regexp
val string_match : regexp -> string -> int -> bool
. . .
val first_chars : string -> int -> string
val last_chars : string -> int -> string
end
Update
(Note that this answer is from 2013. Recent revisions of OCaml provide a toplevel directive to show a module interface:
# #show_module Str;;
module Str :
sig
type regexp
val regexp : string -> regexp
. . .
val first_chars : string -> int -> string
val last_chars : string -> int -> string
end
So the semi-clever workaround is no longer required.
(There are many new directives. Type #help;; at toplevel to get a list.)
Both utop and ocaml interpreters added the #show directive since a moment. It does exactly what you want, as in the following example :
│ Welcome to utop version 1.19.3 (using OCaml version 4.04.0) │
└──────────────────────────────────────────────────────────────┘
Type #utop_help for help about using utop.
─( 15:12:33 )─< command 0 >──────────────────────────────────────{ counter: 0 }─
utop # #show List;;
module List :
sig
val length : 'a list -> int
val cons : 'a -> 'a list -> 'a list
val hd : 'a list -> 'a
...
val fast_sort : ('a -> 'a -> int) -> 'a list -> 'a list
val sort_uniq : ('a -> 'a -> int) -> 'a list -> 'a list
val merge : ('a -> 'a -> int) -> 'a list -> 'a list -> 'a list
end
PS:i'm using 4.04 version but i know that it also works for 4.03.> and maybe before that too.
In OCaml toplevel version 4.04.0, the trick of defining a module's synonym works no more:
# module L = List;;
module L = List
#
but you can use the include directive:
# module L = struct include List end;;
module L :
sig
val length : 'a list -> int
val cons : 'a -> 'a list -> 'a list
val hd : 'a list -> 'a
val tl : 'a list -> 'a list
val nth : 'a list -> int -> 'a
val rev : 'a list -> 'a list
...
val sort_uniq : ('a -> 'a -> int) -> 'a list -> 'a list
val merge : ('a -> 'a -> int) -> 'a list -> 'a list -> 'a list
end
#
I'm a OCaml newbie. I'm playing around with "hello world" type snippets and came across this situation.
Here's a session with the interpreter with some extra comments:
# let average a b =
(a +. b) /. 2.;;
val average : float -> float -> float = <fun>
# average 1. 4.;;
- : float = 2.5
# string_of_float (average 1. 4.);;
- : string = "2.5"
(* this fails...*)
# let _ = Printf.printf (string_of_float (average 1. 4.));;
Error: This expression has type string but an expression was expected of type
('a, out_channel, unit) format =
('a, out_channel, unit, unit, unit, unit) format6
(* yet this works *)
# "hello!";;
- : string = "hello!"
# let _ = Printf.printf "hello!";;
hello!- : unit = ()
(* another failed attempt *)
# let s = string_of_float (average 1. 4.);;
val s : string = "2.5"
# s;;
- : string = "2.5"
# let _ = Printf.printf s;;
Error: This expression has type string but an expression was expected of type
('a, out_channel, unit) format =
('a, out_channel, unit, unit, unit, unit) format6
(* and this also works?? *)
# let _ = Printf.printf "2.5";;
2.5- : unit = ()
So here's the situation. string_of_float (average 1. 4.) returns a string,
just as "hello!" does. When I give "hello!" into Printf.printf, it works
as expected. When I give string_of_float (average 1. 4.) to Printf.printf it fails and tells
me it expected didn't expect a string but that other weird type. But why do "hello!" and "2.5" work then?
What's going on?
There is a kind of "overloading" of the meaning of string literals in OCaml. At compile time, they can either be interpreted as a string, or as a format (which are completely different things in the type system), depending on what the type-checker thinks. If it decides that it should be a format, then the format string is parsed directly at compile-time (that is why it is able to type-check the arguments to printf at compile time). (Unlike C, which parses the string at runtime.) However, there is no simple way to convert from a string into a format at runtime. So when you see Printf.printf "2.5", the "2.5" is actually not a string, but as a special format type that was parsed at compile-time. That is why you cannot substitute a string for it.
On an unrelated note, if you just want to print a string, you might want to use print_string (or print_endline if you want a newline).
Printf.printf "%s" anyStringExpr
will work. The first argument to printf is somewhat magical. (Others will fill in the details.)
I have a type class Atomic, which defines functions for converting certain types to/from a wrapper value (Atom). I'd like to define a QuickCheck property which states: "for all instances of Atomic, any value may be stored and retrieved safely". The property looks like this:
class Atomic a where
toAtom :: a -> Atom
fromAtom :: Atom -> Maybe a
prop_AtomIdentity x = fromAtom (toAtom x) == Just x
However, if I just try to run that property through QuickCheck, it just picks one instance (Bool) and tests it. I'm currently working around that by defining type signatures for each supported atomic type in the test list, but this is verbose and error-prone:
containerTests =
[ run (prop_AtomIdentity :: Bool -> Bool)
, run (prop_AtomIdentity :: Word8 -> Bool)
, run (prop_AtomIdentity :: String -> Bool)
{- etc -} ]
I'm trying to define a function which will do this automatically:
forallAtoms :: (Atomic a, Show a) => (a -> Bool) -> [TestOptions -> IO TestResult]
forallAtoms x =
[ run (x :: Bool -> Bool)
, run (x :: Word8 -> Bool)
, run (x :: String -> Bool)
{- etc -} ]
containerTests = forallAtoms prop_AtomIdentity
But it fails with a typecheck error:
Tests/Containers.hs:33:0:
Couldn't match expected type `Word8' against inferred type `String'
In the first argument of `run', namely `(x :: Word8 -> Bool)'
In the expression: run (x :: Word8 -> Bool)
In the expression:
[run (x :: Bool -> Bool), run (x :: Word8 -> Bool),
run (x :: String -> Bool)]
Is there a better way to test a QC property against multiple types? If not, can forallAtoms be made to work or is that not supported by the type system?
I cannot compile your code, so ... blind shot:
try
forallAtoms :: (forall a. (Atomic a, Show a) => a -> Bool) -> [TestOptions -> IO TestResult]
as a type signature. This needs the -XRankNTypes language extension.
The problem you have, as I see it, is that GHC tries to find one type to insert for a in x :: (a -> Bool) for the entire function scope, but you already give three different there.