Re: [Haskell-cafe] Re: [Haskell-beginners] map question
[snip] Not a hack, a solution. A consistent one. Look: (`foldl` 0) (`-` 2) Don't they look exactly the same? [snip] These look the same too (and *are* consistent): (f a b) (+ a b) But it's not Haskell.. IMO conflating binary minus and unary minus is not consistent. Something I wonder from time to time if it would be a good thing or not is to allow a `f g` b to mean f g a b (so map (`f g` b) as would legal too). Cheers, Thu ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: [Haskell-beginners] map question
Something I wonder from time to time if it would be a good thing or not is to allow a `f g` b to mean f g a b You don't have to wonder: http://www.haskell.org/haskellwiki/Infix_expressions Granted, you have to use different characters rather than the backquote. On the other hand, you can play with infix expressions right now, on any Haskell98 system. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: [Haskell-beginners] map question
Jason Dagit dagit at codersbase.com writes: On Mon, Oct 19, 2009 at 5:53 PM, Will Ness will_n48 at yahoo.com wrote: You think of functions, where domain matters (for purists?). In syntax only the result matter, does it read? Does it have an intended meaning? How is it a mistake if it expresses what I intended? Both 3 `-` 2 and curry fst `foldl` 0 are exactly the same - expressions with infix operator, read in the same way, interpreted in the same way. In the first case the backticks are made superfluous by Haskell reader for our convinience; but they shouldn't be made illegal. Why should they be? Don't you mean 3 `(-)` 2? I'm pretty sure -, without the parens is infix and (-) is prefix. So it seems to me that you need the brackets for this to be consistent.Jason You absolutely right, in current syntax that also would only be consistent, yet is illegal also. But I propose to augment the syntax by allowing symbolic ops in backticks to stand for themselves. When I see `op`, for me, it says: infix op. So `+` would also say, infix +. (`- ` 2) would finally become possible. It would read: treat - as infix binary and make a flip section out of it. Just as it does for an alphanumeric identifier in (`op` 2). Without backticks, symbolic ops are also treated as infix by default, but that's just convinience. Anyway I guess all the points in this discussion have been made, and it's just a matter of taste. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: [Haskell-beginners] map question
It's worth remembering that APL and SML, amongst others, distinguish between the sign used for a negative literal (¯1 in APL, ~1 in SML) and the sign used for subtraction (the hyphen/minus in both of them). It doesn't seem to be a hard thing to get your head around in practice. From having worked on a Prolog system, I can tell you that the fact that -1 is a single token except when it isn't, yet -X is always two, caused headaches for implementors and confusion for users. In Smalltalk, -1 is a number, but x-1 is three tokens, not two. (You have to keep track of what the previous token was to tell what to do.) If I were making suggestions for Haskell' (other than please, pretty please with knobs on, let me keep n+k), one of them would be to introduce the character U+00AF (chosen because it's 8859-1, -4, -8, -9, and -15 at least) as a unary minus sign, allowing it to be used for exponent signs as well, so that ¯x - ¯1.0e¯10 is allowed. Then Haskell'' could remove the unary - . In the mean time, the unary - / binary - issue is something you run into hard ONCE, and then avoid easily enough, not unlike forgetting the back-ticks in x `div` y. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: [Haskell-beginners] map question
minh thu wrote: Something I wonder from time to time if it would be a good thing or not is to allow a `f g` b to mean f g a b This comes up from time to time, though it is often met with stern disapproval because it can easily lead to loss of clarity. There is a valid alternative, though it's not quite as pretty: ($a) (f g...) b c... == f g... a b c... With even less prettiness, this can also be generalized for other numbers of prefix arguments: (($a) f) b c... == f a b c... ( ($b) $($a) f) c d... == f a b c d... (($c) $($b) $($a) f) d e... == f a b c d e... And if anyone wanted to use this sort of pattern frequently, I'm sure there's some decent way to clean it up ala Matt Hellige's pointless trick[1] or ala Oleg's polyvariadic trick[2]. [1] http://matt.immute.net/content/pointless-fun [2] http://okmij.org/ftp/Haskell/polyvariadic.html -- Live well, ~wren ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: [Haskell-beginners] map question
wren ng thornton wren at freegeek.org writes: Will Ness wrote: (`foldl`2) works. (`-`2) should too. The `` syntax is for converting lexical identifiers into infix operators. Symbolic identifiers are already infix, which is why `` So it would be a no-op then. Why make it illegal? Just because it makes writing the scanner easier is no answer. doesn't work for them. If we introduced this then those striving for consistency would be right in requesting that this pattern be allowed for all symbolic operators. I for one am opposed to introducing superfluous syntax for duplicating the current ability to write things in the same ways. This syntax already exists. The '`' symbol is non-collating already, so using it for symbol chars doesn't change anything (it's not that it can be a part of some name, right?). To turn an infix op into an infix op is an id operation, made illegal artificially at the scan phase after a successful lex (or whatever). Finally enabling the missing functionality which is a common stumbling block for every beginner is hardly duplicating. Attack the underlying problem, don't introduce hacks to cover up broken hacks. This isn't C++. The underlying problem is a broken scanner where it can't distinguish between a binary op and a number read syntax. Op names are collated symbol chars, and one of the symbols, -, is also a valid number prefix. So, allow for a clues from programmer to detach it from the number: backticks separate it from the following numeric chars, preventing it from sticking to them. And by itself, it forms an op, a binary one. Not a hack, a solution. A consistent one. Look: (`foldl` 0) (`-` 2) Don't they look exactly the same? Why wouldn't it be made legal? Show me one inconsistency it introduces. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: [Haskell-beginners] map question
On Mon, Oct 19, 2009 at 5:34 PM, Will Ness will_...@yahoo.com wrote: This syntax already exists. The '`' symbol is non-collating already, so using it for symbol chars doesn't change anything (it's not that it can be a part of some name, right?). To turn an infix op into an infix op is an id operation, made illegal artificially at the scan phase after a successful lex (or whatever). If I've accidentally applied syntax meant for a prefix operator to an infix operator, *I want the compiler to tell me*, and not to silently accept my mistake. Not a hack, a solution. A consistent one. Look: (`foldl` 0) (`-` 2) Don't they look exactly the same? No, because the latter is applying prefix-to-infix syntax to an infix operator. It's understood that non-alphanumerics are infix by default, and I want the compiler to scream at me if I try to use one where it expected a prefix op. Why wouldn't it be made legal? Show me one inconsistency it introduces. You've said that you want to be able to do this for the sole case of the - (minus-sign) operator: Operators are great because they make our intent visible, immediately apparent. Long words' meaning, like subtract's, is not immediately apparent, and they break consistency. Not everyone's first language in life was English, you see. I don't buy this rationale. Haskell has plenty of English words as function names all over the place; if you can't handle subtract, how are you handling Haskell at all? Sure, the minus-sign issue is a wart, but it's less awkward than the solution you propose for a problem I doubt you really have. :-) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: [Haskell-beginners] map question
Tom Tobin korpios at korpios.com writes: On Mon, Oct 19, 2009 at 5:34 PM, Will Ness will_n48 at yahoo.com wrote: This syntax already exists. The '`' symbol is non-collating already, so using it for symbol chars doesn't change anything (it's not that it can be a part of some name, right?). To turn an infix op into an infix op is an id operation, made illegal artificially at the scan phase after a successful lex (or whatever). If I've accidentally applied syntax meant for a prefix operator to an infix operator, *I want the compiler to tell me*, and not to silently accept my mistake. You don't apply sytax, you write it. You think of functions, where domain matters (for purists?). In syntax only the result matter, does it read? Does it have an intended meaning? How is it a mistake if it expresses what I intended? Both 3 `-` 2 and curry fst `foldl` 0 are exactly the same - expressions with infix operator, read in the same way, interpreted in the same way. In the first case the backticks are made superfluous by Haskell reader for our convinience; but they shouldn't be made illegal. Why should they be? I truly don't understand the resistance to this idea. :) Why wouldn't it be made legal? Show me one inconsistency it introduces. You've said that you want to be able to do this for the sole case of the - (minus-sign) operator: This is not an inconsistence. Plus, if we were to take this idea of using backticks as names delimeters to the extreme, it could even allow us to use such identifiers as `left-fold` or `right-fold` in infix position, and (`left-fold`) by itself. Although that seems not such a good idea. Operators are great because they make our intent visible, immediately apparent. Long words' meaning, like subtract's, is not immediately apparent, and they break consistency. Not everyone's first language in life was English, you see. I don't buy this rationale. Haskell has plenty of English words as function names all over the place; if you can't handle subtract, how are you handling Haskell at all? Sure, the minus-sign issue is a wart, but it's less awkward than the solution you propose for a problem I doubt you really have. When I see `++` I don't need to think _at_all_. When I see `concatenate` or some such, I do - even if for a briefest of moments. It is _less_ convinient both to read and _write_, don't you agree? I don't see my proposal as awkward at all. On the contrary, to me it looks natural and consistent with the other uses of this device in the language. It is this asymmetry that bothers me with the (-) issue, I just want the balance restored. But it is a matter of taste of course. Or obsessing over minutiae. :) Oh well. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: [Haskell-beginners] map question
Will Ness wrote: wren ng thornton writes: Attack the underlying problem, don't introduce hacks to cover up broken hacks. This isn't C++. The underlying problem is a broken scanner where it can't distinguish between a binary op and a number read syntax. The underlying problem is that (1) people don't want normal whitespace to change the meaning of code, (2) they don't want to disallow negative literals, and (3) they want to use the same symbolic operator for negation and subtraction, but these three goals cannot all be satisfied simultaneously. The current resolution is to hack at the parser in order to make things mostly work. But this hack is insufficient, as argued by the OP. The proposed solution was to introduce new syntax complicating the language by explaining how 1-2 and 1`-`2 are the same thing (either repeated for all other symbolic operators, or exceptional to the subtraction operator, and ugly by either approach). But why should we introduce all this syntactic complexity which needs explaining to newbies and only makes the wart more visible? The proper solution is not to introduce syntactic hackery on top of the parser hackery, the proper solution is to either come up with a better parser hack or to sacrifice one of the three incompatible goals. Not a hack, a solution. A consistent one. Look: (`foldl` 0) (`-` 2) Don't they look exactly the same? Not to me they don't. Symbolic and lexical operators are treated differently in Haskell. Considering all the places where they're treated differently, I see no compelling reason to think they should be considered similar here. -- Live well, ~wren ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: [Haskell-beginners] map question
On Mon, Oct 19, 2009 at 5:53 PM, Will Ness will_...@yahoo.com wrote: Tom Tobin korpios at korpios.com writes: On Mon, Oct 19, 2009 at 5:34 PM, Will Ness will_n48 at yahoo.com wrote: This syntax already exists. The '`' symbol is non-collating already, so using it for symbol chars doesn't change anything (it's not that it can be a part of some name, right?). To turn an infix op into an infix op is an id operation, made illegal artificially at the scan phase after a successful lex (or whatever). If I've accidentally applied syntax meant for a prefix operator to an infix operator, *I want the compiler to tell me*, and not to silently accept my mistake. You don't apply sytax, you write it. You think of functions, where domain matters (for purists?). In syntax only the result matter, does it read? Does it have an intended meaning? How is it a mistake if it expresses what I intended? Both 3 `-` 2 and curry fst `foldl` 0 are exactly the same - expressions with infix operator, read in the same way, interpreted in the same way. In the first case the backticks are made superfluous by Haskell reader for our convinience; but they shouldn't be made illegal. Why should they be? I truly don't understand the resistance to this idea. :) Don't you mean 3 `(-)` 2? I'm pretty sure -, without the parens is infix and (-) is prefix. So it seems to me that you need the brackets for this to be consistent. Jason ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: [Haskell-beginners] map question
On Sun, Oct 18, 2009 at 5:31 PM, Will Ness will_...@yahoo.com wrote: Luke Palmer lrpalmer at gmail.com writes: Or you could use the subtract function. map (subtract 2) [3,4,5] [1,2,3] I don't want to. I think at about this point, this stopped being an intellectual discussion. Preparing for academic flame war... I don't think syntax sugar is worth it in this case. I do. Operators are great because they make our intent visible, immediately apparent. Long words' meaning, like subtract's, is not immediately apparent, and they break consistency. Not everyone's first language in life was English, you see. (`foldl`2) works. (`-`2) should too. I'll settle for (+(-2)) for now, but it ain't that pretty. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: [Haskell-beginners] map question
Gregory Propf gregorypropf at yahoo.com writes: I actually meant it as sort of a joke but maybe it's not after all. Seriously though, using anything non-ASCII in source code is a bad idea, because there are lots of fonts and editors in the world. It seems natural to me to have (`-`2) stand for (flip (-) 2), if only that would be made legal syntax, just as (`foldl`0) stands for (flip (foldl) 0). Supposedly there is no reason to write (`:`[]) since : is already an infix operator, but making it a no-op wouldn't hurt, and would give us a benefit of being able finally to write the binary-minus flip-section in a visually apparent way. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: [Haskell-beginners] map question
On Sun, Oct 18, 2009 at 4:47 PM, Will Ness will_...@yahoo.com wrote: Gregory Propf gregorypropf at yahoo.com writes: I actually meant it as sort of a joke but maybe it's not after all. Seriously though, using anything non-ASCII in source code is a bad idea, because there are lots of fonts and editors in the world. It seems natural to me to have (`-`2) stand for (flip (-) 2), if only that would be made legal syntax, just as (`foldl`0) stands for (flip (foldl) 0). Or you could use the subtract function. map (subtract 2) [3,4,5] [1,2,3] I don't think syntax sugar is worth it in this case. Luke ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: [Haskell-beginners] map question
Luke Palmer lrpalmer at gmail.com writes: Or you could use the subtract function. map (subtract 2) [3,4,5] [1,2,3] I don't want to. I don't think syntax sugar is worth it in this case. I do. Operators are great because they make our intent visible, immediately apparent. Long words' meaning, like subtract's, is not immediately apparent, and they break consistency. Not everyone's first language in life was English, you see. (`foldl`2) works. (`-`2) should too. I'll settle for (+(-2)) for now, but it ain't that pretty. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: [Haskell-beginners] map question
Will Ness wrote: Luke Palmer lrpalmer at gmail.com writes: Or you could use the subtract function. map (subtract 2) [3,4,5] [1,2,3] I don't want to. I don't think syntax sugar is worth it in this case. I do. Operators are great because they make our intent visible, immediately apparent. Long words' meaning, like subtract's, is not immediately apparent, and they break consistency. Not everyone's first language in life was English, you see. I'm with Luke on this one. It's a shame that negation uses the same symbolic identifier as subtraction, but introducing this new sugar only serves to make things more complex than they already are. If anything, negation should be moved to using a different identifier to remove the current ambiguity (as is done in some other languages). (`foldl`2) works. (`-`2) should too. The `` syntax is for converting lexical identifiers into infix operators. Symbolic identifiers are already infix, which is why `` doesn't work for them. If we introduced this then those striving for consistency would be right in requesting that this pattern be allowed for all symbolic operators. I for one am opposed to introducing superfluous syntax for duplicating the current ability to write things in the same ways. Attack the underlying problem, don't introduce hacks to cover up broken hacks. This isn't C++. -- Live well, ~wren ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: [Haskell-beginners] map question
Gregory Propf gregorypr...@yahoo.com writes: Heh, perhaps we should petition to have a new computer key and symbol added to the world's way of writing maths, something like maybe a downward angled slash to mean prefix (-) Or just use 'negate' and 'subtract'? -k -- If I haven't seen further, it is by standing in the footprints of giants ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: [Haskell-beginners] map question
Ketil Malde ke...@malde.org writes: Gregory Propf gregorypr...@yahoo.com writes: Heh, perhaps we should petition to have a new computer key and symbol added to the world's way of writing maths, something like maybe a downward angled slash to mean prefix (-) Or just use 'negate' and 'subtract'? Well, now that ghc accepts unicode characters in programme source, we could ask that ¬ (NOT SIGN, U+00AC) be recategorised as an identifier character and use that (as a simple function name) for negation and lose the wart altogether. class Negatable t where ¬ :: t - t (and as a side effect we could have identifiers like slightly¬dodgy). Or, if we want to make things look even nicer, make ‐ (HYPHEN, U+2010) an identifier character and use − (MINUS SIGN, U+2212) for the infix operator. Now we could have hyphenated‐identifiers too. I think this second option would be the ㊣ (CORRECT, U+32A3) thing to do, though editors and so on would have to be changed to make the distinction readily visible. I think it's Friday, but I'm not entirely sure this is silly. -- Jón Fairbairn jon.fairba...@cl.cam.ac.uk ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: [Haskell-beginners] map question
On 18 Sep 2009, at 04:32, Gregory Propf wrote: Heh, perhaps we should petition to have a new computer key and symbol added to the world's way of writing maths, something like maybe a downward angled slash to mean prefix (-) Such a symbol already exists, but isn't in the ASCII set: (-) (unicode 0x2D) hyphen minus and (‐) (unicode 0x2010) hyphen are not the same as (−) (unicode 0x2200) minus sign notably also, not the same as ‒, –, — and ― (figure dash, en- dash, em-dash and horizontal bar). Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: [Haskell-beginners] map question
I actually meant it as sort of a joke but maybe it's not after all. Among the many benefits, think of all the delightful conspiracy theories such a change would spawn - even our math isn't safe now!, Save the minus sign!. --- On Fri, 9/18/09, Jon Fairbairn jon.fairba...@cl.cam.ac.uk wrote: From: Jon Fairbairn jon.fairba...@cl.cam.ac.uk Subject: [Haskell-cafe] Re: [Haskell-beginners] map question To: haskell-cafe@haskell.org Date: Friday, September 18, 2009, 2:09 AM Ketil Malde ke...@malde.org writes: Gregory Propf gregorypr...@yahoo.com writes: Heh, perhaps we should petition to have a new computer key and symbol added to the world's way of writing maths, something like maybe a downward angled slash to mean prefix (-) Or just use 'negate' and 'subtract'? Well, now that ghc accepts unicode characters in programme source, we could ask that ¬ (NOT SIGN, U+00AC) be recategorised as an identifier character and use that (as a simple function name) for negation and lose the wart altogether. class Negatable t where ¬ :: t - t (and as a side effect we could have identifiers like slightly¬dodgy). Or, if we want to make things look even nicer, make ‐ (HYPHEN, U+2010) an identifier character and use − (MINUS SIGN, U+2212) for the infix operator. Now we could have hyphenated‐identifiers too. I think this second option would be the ㊣ (CORRECT, U+32A3) thing to do, though editors and so on would have to be changed to make the distinction readily visible. I think it's Friday, but I'm not entirely sure this is silly. -- Jón Fairbairn jon.fairba...@cl.cam.ac.uk ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: [Haskell-beginners] map question
Remember that there is asymmetry between (+) and (-). The former has the commutative property and the latter does not so: (+) 3 4 = 7 and (+) 4 3 = 7 but (-) 3 4 = -1 and (-) 4 3 = 1 --- On Thu, 9/17/09, Tom Doris tomdo...@gmail.com wrote: From: Tom Doris tomdo...@gmail.com Subject: Re: [Haskell-beginners] map question To: Joost Kremers joostkrem...@fastmail.fm Cc: beginn...@haskell.org Date: Thursday, September 17, 2009, 6:06 AM This works: map (+ (-1)) [1,2,3,4] 2009/9/17 Joost Kremers joostkrem...@fastmail.fm Hi all, I've just started learning Haskell and while experimenting with map a bit, I ran into something I don't understand. The following commands do what I'd expect: Prelude map (+ 1) [1,2,3,4] [2,3,4,5] Prelude map (* 2) [1,2,3,4] [2,4,6,8] Prelude map (/ 2) [1,2,3,4] [0.5,1.0,1.5,2.0] Prelude map (2 /) [1,2,3,4] [2.0,1.0,0.,0.5] But I can't seem to find a way to get map to substract 1 from all members of the list. The following form is the only one that works, but it doesn't give the result I'd expect: Prelude map ((-) 1) [1,2,3,4] [0,-1,-2,-3] I know I can use an anonymous function, but I'm just trying to understand the result here... I'd appreciate any hints to help me graps this. TIA Joost -- Joost Kremers, PhD University of Frankfurt Institute for Cognitive Linguistics Grüneburgplatz 1 60629 Frankfurt am Main, Germany ___ Beginners mailing list beginn...@haskell.org http://www.haskell.org/mailman/listinfo/beginners -Inline Attachment Follows- ___ Beginners mailing list beginn...@haskell.org http://www.haskell.org/mailman/listinfo/beginners ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: [Haskell-beginners] map question
(-) happens to be the only prefix operator in haskell, it also an infix operator. so: 4 - 2 2 -3 -3 ((-) 5) 3 -- note that in this case (-) is treated like any regular function so 5 is the first parameter 2 (5 - ) 3 2 (-5 ) -5 (flip (-) 5) 3 -2 It's a little wart brought about by the ambiguity in common mathematical syntax. If you play around in ghci you should get the hang of it pretty quick. - Job On Thu, Sep 17, 2009 at 11:08 AM, Gregory Propf gregorypr...@yahoo.comwrote: Remember that there is asymmetry between (+) and (-). The former has the commutative property and the latter does not so: (+) 3 4 = 7 and (+) 4 3 = 7 but (-) 3 4 = -1 and (-) 4 3 = 1 --- On *Thu, 9/17/09, Tom Doris tomdo...@gmail.com* wrote: From: Tom Doris tomdo...@gmail.com Subject: Re: [Haskell-beginners] map question To: Joost Kremers joostkrem...@fastmail.fm Cc: beginn...@haskell.org Date: Thursday, September 17, 2009, 6:06 AM This works: map (+ (-1)) [1,2,3,4] 2009/9/17 Joost Kremers joostkrem...@fastmail.fmhttp://mc/compose?to=joostkrem...@fastmail.fm Hi all, I've just started learning Haskell and while experimenting with map a bit, I ran into something I don't understand. The following commands do what I'd expect: Prelude map (+ 1) [1,2,3,4] [2,3,4,5] Prelude map (* 2) [1,2,3,4] [2,4,6,8] Prelude map (/ 2) [1,2,3,4] [0.5,1.0,1.5,2.0] Prelude map (2 /) [1,2,3,4] [2.0,1.0,0.,0.5] But I can't seem to find a way to get map to substract 1 from all members of the list. The following form is the only one that works, but it doesn't give the result I'd expect: Prelude map ((-) 1) [1,2,3,4] [0,-1,-2,-3] I know I can use an anonymous function, but I'm just trying to understand the result here... I'd appreciate any hints to help me graps this. TIA Joost -- Joost Kremers, PhD University of Frankfurt Institute for Cognitive Linguistics Grüneburgplatz 1 60629 Frankfurt am Main, Germany ___ Beginners mailing list beginn...@haskell.org http://mc/compose?to=beginn...@haskell.org http://www.haskell.org/mailman/listinfo/beginners -Inline Attachment Follows- ___ Beginners mailing list beginn...@haskell.org http://mc/compose?to=beginn...@haskell.org http://www.haskell.org/mailman/listinfo/beginners ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: [Haskell-beginners] map question
2009/9/17 Joost Kremers joostkrem...@fastmail.fm Hi all, I've just started learning Haskell and while experimenting with map a bit, I ran into something I don't understand. The following commands do what I'd expect: Prelude map (+ 1) [1,2,3,4] [2,3,4,5] Prelude map (* 2) [1,2,3,4] [2,4,6,8] Prelude map (/ 2) [1,2,3,4] [0.5,1.0,1.5,2.0] Prelude map (2 /) [1,2,3,4] [2.0,1.0,0.,0.5] But I can't seem to find a way to get map to substract 1 from all members of the list. The following form is the only one that works, but it doesn't give the result I'd expect: Prelude map ((-) 1) [1,2,3,4] [0,-1,-2,-3] I know I can use an anonymous function, but I'm just trying to understand the result here... I'd appreciate any hints to help me graps this. TIA Joost The reason that map (-1) [1,2,3,4] doesn't work as you'd expect it to is that - is ambiguous in Haskell (some may disagree). -1 means -1 in Haskell, i.e. negative 1, not the function that subtracts 1 from its argument. (-) 1 is the function that subtracts its argument from 1, which is not what you were looking for either! You're looking for the function that subtracts 1 from its argument, which is `subtract 1'. Prelude map (subtract 1) [1..4] [0,1,2,3] Note that `subtract' is just another name for `flip (-)', i.e. subtraction with its argument in reverse order. -- Deniz Dogan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: [Haskell-beginners] map question
Heh, perhaps we should petition to have a new computer key and symbol added to the world's way of writing maths, something like maybe a downward angled slash to mean prefix (-) :) --- On Thu, 9/17/09, Job Vranish jvran...@gmail.com wrote: From: Job Vranish jvran...@gmail.com Subject: Re: [Haskell-cafe] Re: [Haskell-beginners] map question To: Gregory Propf gregorypr...@yahoo.com Cc: Tom Doris tomdo...@gmail.com, Haskell-Cafe haskell-cafe@haskell.org, joostkrem...@fastmail.fm Date: Thursday, September 17, 2009, 9:04 AM (-) happens to be the only prefix operator in haskell, it also an infix operator. so: 4 - 2 2 -3 -3 ((-) 5) 3 -- note that in this case (-) is treated like any regular function so 5 is the first parameter 2 (5 - ) 3 2 (-5 ) -5 (flip (-) 5) 3 -2 It's a little wart brought about by the ambiguity in common mathematical syntax. If you play around in ghci you should get the hang of it pretty quick. - Job On Thu, Sep 17, 2009 at 11:08 AM, Gregory Propf gregorypr...@yahoo.com wrote: Remember that there is asymmetry between (+) and (-). The former has the commutative property and the latter does not so: (+) 3 4 = 7 and (+) 4 3 = 7 but (-) 3 4 = -1 and (-) 4 3 = 1 --- On Thu, 9/17/09, Tom Doris tomdo...@gmail.com wrote: From: Tom Doris tomdo...@gmail.com Subject: Re: [Haskell-beginners] map question To: Joost Kremers joostkrem...@fastmail.fm Cc: beginn...@haskell.org Date: Thursday, September 17, 2009, 6:06 AM This works: map (+ (-1)) [1,2,3,4] 2009/9/17 Joost Kremers joostkrem...@fastmail.fm Hi all, I've just started learning Haskell and while experimenting with map a bit, I ran into something I don't understand. The following commands do what I'd expect: Prelude map (+ 1) [1,2,3,4] [2,3,4,5] Prelude map (* 2) [1,2,3,4] [2,4,6,8] Prelude map (/ 2) [1,2,3,4] [0.5,1.0,1.5,2.0] Prelude map (2 /) [1,2,3,4] [2.0,1.0,0.,0.5] But I can't seem to find a way to get map to substract 1 from all members of the list. The following form is the only one that works, but it doesn't give the result I'd expect: Prelude map ((-) 1) [1,2,3,4] [0,-1,-2,-3] I know I can use an anonymous function, but I'm just trying to understand the result here... I'd appreciate any hints to help me graps this. TIA Joost -- Joost Kremers, PhD University of Frankfurt Institute for Cognitive Linguistics Grüneburgplatz 1 60629 Frankfurt am Main, Germany ___ Beginners mailing list beginn...@haskell.org http://www.haskell.org/mailman/listinfo/beginners -Inline Attachment Follows- ___ Beginners mailing list beginn...@haskell.org http://www.haskell.org/mailman/listinfo/beginners ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
