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https://codeberg.org/mhwombat/nix-book.git
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expanded
This commit is contained in:
Amy de Buitléir
parent
d0be400f85
commit
96f65dd5a5
11 changed files with 102 additions and 7 deletions
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@ -4,6 +4,7 @@ The conditional construct in Nix is an _expression_, not a _statement_.
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Since expressions must have values in all cases, you must specify both the `then` and the `else` branch.
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[source]
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.Example
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....
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nix-repl> a = 7
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@ -3,6 +3,7 @@
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A `let` expression defines a value with a local scope.
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[source]
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.Example
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....
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nix-repl> let x = 3; in x*x
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9
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@ -15,6 +16,7 @@ You can also nest `let` expressions.
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The previous expression is equivalent to the following.
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[source]
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.Example
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....
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nix-repl> let x = 3; in let y = 2; in x*x + y
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11
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@ -25,6 +27,7 @@ nix-repl> let x = 3; in let y = 2; in x*x + y
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A variable defined inside a `let` expression will "shadow" an outer variable with the same name.
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[source]
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.Example
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....
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nix-repl> x = 100
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@ -40,6 +43,7 @@ A variable in a let expression can refer to another variable in the expression.
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This is similar to how recursive attribute sets work.
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[source]
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.Example
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....
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nix-repl> let x = 3; y = x + 1; in x*y
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12
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@ -5,6 +5,7 @@
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Lists can be concatenated using the `++` operator.
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[source]
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.Example
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....
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nix-repl> [ 1 2 3 ] ++ [ "apple" "banana" ]
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[ 1 2 3 "apple" "banana" ]
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@ -20,6 +21,7 @@ For more information on these and other built-in functions, see the Nix Manual
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Testing if an element appears in a list.
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[source]
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.Example
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....
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nix-repl> fruit = [ "apple" "banana" "cantaloupe" ]
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@ -34,6 +36,7 @@ Selecting an item from a list by index.
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The first element in a list has index `0`.
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[source]
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.Example
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....
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nix-repl> builtins.elemAt fruit 0
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"apple"
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@ -45,6 +48,7 @@ nix-repl> builtins.elemAt fruit 2
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Determining the number of elements in a list.
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[source]
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.Example
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....
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nix-repl> builtins.length fruit
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3
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@ -53,6 +57,7 @@ nix-repl> builtins.length fruit
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Accessing the first element of a list.
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[source]
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.Example
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....
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nix-repl> builtins.head fruit
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"apple"
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@ -61,6 +66,7 @@ nix-repl> builtins.head fruit
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Dropping the first element of a list.
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[source]
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.Example
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....
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nix-repl> builtins.tail fruit
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[ "banana" "cantaloupe" ]
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@ -73,6 +79,7 @@ but the following examples should be somewhat self-explanatory.
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Using a function to filter (select elements from) a list.
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[source]
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.Example
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....
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nix-repl> numbers = [ 1 3 6 8 9 15 25 ]
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@ -85,6 +92,7 @@ nix-repl> builtins.filter isBig numbers # get just the "big" numbers
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Applying a function to all the elements in a list.
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[source]
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.Example
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....
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nix-repl> double = n: 2*n # multiply by two
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@ -5,6 +5,7 @@
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The usual arithmetic operators are provided.
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[source]
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.Example
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....
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nix-repl> 1 + 2 # addition
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3
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@ -29,6 +30,7 @@ you can get unexpected results when you omit spaces around operators.
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Consider the following example.
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[source]
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.Example
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....
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nix-repl> 6/2
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/home/amy/codeberg/nix-book/6/2
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@ -38,13 +40,14 @@ What happened?
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Let's use the `:t` command to find out the type of the expression.
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[source]
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.Example
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....
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nix-repl> :t 6/2
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a path
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....
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If an expression can be interpreted as a path, Nix will do so.
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This is useful, because paths are _far_ more commonly used in Nix expressions
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This is useful, because paths are _far_ more commonly used in Nix expressions
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than arithmetic operators.
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In this case, Nix interpreted `6/2` as a relative path from the current directory,
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which in the above example was `/home/amy/codeberg/nix-book`.
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@ -52,6 +55,7 @@ which in the above example was `/home/amy/codeberg/nix-book`.
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Adding a space after the `/` operator produces the expected result.
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[source]
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.Example
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....
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nix-repl> 6/ 2
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3
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@ -66,6 +70,7 @@ Numbers without a decimal point are assumed to be integers.
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To ensure that a number is interpreted as a floating-point value, add a decimal point.
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[source]
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.Example
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....
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nix-repl> :t 5
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an integer
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@ -81,6 +86,7 @@ In the example below, the first expression results in integer division (rounding
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while the second produces a floating-point result.
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[source]
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.Example
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....
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nix-repl> 5 / 3
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1
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@ -6,6 +6,7 @@ To refer to a file or directory relative to the current directory, prefix it wit
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You can specify the current directory as `./.`
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[source]
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.Example
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....
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nix-repl> ./file.txt
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/home/amy/codeberg/nix-book/file.txt
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@ -19,6 +20,7 @@ nix-repl> ./.
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Paths can be concatenated to produce a new path.
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[source]
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.Example
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....
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nix-repl> /home/wombat + /bin/sh
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/home/wombat/bin/sh
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@ -33,6 +35,7 @@ Relative paths are made absolute when they are parsed, which occurs before conca
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This is why the result in the example below is not `/home/wombat/file.txt`.
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[source]
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.Example
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....
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nix-repl> /home/wombat + ./file.txt
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/home/wombat/home/amy/codeberg/nix-book/file.txt
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@ -44,6 +47,7 @@ nix-repl> /home/wombat + ./file.txt
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A path can be concatenated with a string to produce a new path.
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[source]
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.Example
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....
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nix-repl> /home/wombat + "/file.txt"
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/home/wombat/file.txt
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@ -61,22 +65,28 @@ for more information see https://nixos.org/manual/nix/stable/language/operators#
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== Concatenating a string + a path
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[IMPORTANT]
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====
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Strings can be concatenated with paths, but with a side-effect that may surprise you:
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if the path exists, the file is copied to the Nix store!
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The result is a string, not a path.
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====
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In the example below, the file `file.txt` is copied to `/nix/store/gp8ba25gpwvbqizqfr58jr014gmv1hd8-file.txt`
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(not, as you might expect, to `/home/wombat/nix/store/gp8ba25gpwvbqizqfr58jr014gmv1hd8-file.txt`).
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In the example below, you might expect the result to be `"home/wombat/file.nix"`.
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However, the file `file.txt` is copied to `/nix/store/gp8ba25gpwvbqizqfr58jr014gmv1hd8-file.txt`
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before concatenating it to the string.
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[source]
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.Example
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....
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nix-repl> "/home/wombat" + ./file.txt
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"/home/wombat/nix/store/gp8ba25gpwvbqizqfr58jr014gmv1hd8-file.txt"
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....
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The path must exist.
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When concatenating a string with a path, the path must exist.
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[source]
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.Example
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....
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nix-repl> "/home/wombat" + ./no-such-file.txt
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error (ignored): error: end of string reached
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@ -93,6 +103,7 @@ For more information on these and other built-in functions, see the Nix Manual
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Does the path exist?
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[source]
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.Example
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....
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nix-repl> builtins.pathExists ./index.html
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true
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@ -104,6 +115,7 @@ false
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Get a list of the files in a directory, along with the type of each file.
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[source]
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.Example
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....
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nix-repl> builtins.readDir ./.
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{ ".envrc" = "regular"; ".git" = "directory"; ".gitignore" = "regular"; Makefile = "regular"; images = "directory"; "index.html" = "regular"; "shell.nix" = "regular"; source = "directory"; themes = "directory"; "wombats-book-of-nix.pdf" = "regular"; }
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@ -112,6 +124,7 @@ nix-repl> builtins.readDir ./.
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Read the contents of a file into a string.
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[source]
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.Example
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....
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nix-repl> builtins.readFile ./.envrc
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"use nix\n"
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@ -41,4 +41,16 @@ The following commands are available:
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A command that is useful to beginners is `:t`, which tells you the type of an expression.
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[source]
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.Example
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....
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nix-repl> :t { abc = true; "123" = false; }
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a set
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nix-repl> f = x: y: x * y
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nix-repl> :t f
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a function
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....
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Note that the command to exit the REPL is `:q` (or `:quit` if you prefer).
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@ -1,10 +1,38 @@
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= String operations
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== String comparison
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Nix provides the usual lexicographic comparison operations.
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[source]
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.Example
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....
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nix-repl> "apple" == "banana" # equality
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false
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nix-repl> "apple" != "banana" # inequality
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true
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nix-repl> "apple" < "banana" # comes alphabetically before?
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true
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nix-repl> "apple" <= "banana" # equal or comes alphabetically before?
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true
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nix-repl> "apple" > "banana" # comes alphabetically after?
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false
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nix-repl> "apple" >= "banana" # equal or comes alphabetically after?
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false
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....
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== String concatenation
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String concatenation uses the `+` operator.
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[source]
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.Example
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....
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nix-repl> "Hello, " + "world!"
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"Hello, world!"
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@ -15,6 +43,7 @@ nix-repl> "Hello, " + "world!"
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You can use the `${_variable_}` syntax to insert the value of a variable within a string.
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[source]
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.Example
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....
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nix-repl> name = "Wombat"
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@ -28,6 +57,7 @@ You cannot mix numbers and strings.
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Earlier we set `a = 7`, so the following expression fails.
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[source]
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.Example
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....
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nix-repl> "My favourite number is ${a}."
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error:
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@ -40,7 +70,7 @@ error:
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error: cannot coerce an integer to a string
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....
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Nix does provide functions for converting between types; we'll see these in the
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Nix does provide functions for converting between types; we'll see these in the
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<<#convert-to-string,next section>>.
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====
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@ -54,6 +84,7 @@ For more information on these and other built-in functions, see the Nix Manual
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How long is this string?
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[source]
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.Example
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....
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nix-repl> builtins.stringLength "supercalifragilisticexpialidocious"
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34
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@ -63,6 +94,7 @@ Given a starting position and a length, extract a substring.
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The first character of a string has index `0`.
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[source]
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.Example
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....
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nix-repl> builtins.substring 3 6 "hayneedlestack"
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"needle"
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@ -72,6 +104,7 @@ nix-repl> builtins.substring 3 6 "hayneedlestack"
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Convert an expression to a string.
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[source]
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.Example
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....
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nix-repl> builtins.toString 7
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"7"
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@ -94,11 +94,22 @@ attribute sets are simply called "sets".
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[#type-lambda]
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== Functions
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We'll learn how to write functions later in this chapter.
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We'll learn how to write functions in <<#functions>>.
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For now, note that functions are "first-class values",
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meaning that they can be treated like any other data type.
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For example, a function can be assigned to a variable, appear as an element in a list,
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or be associated with a key in an attribute set.
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be associated with a key in an attribute set, or be passed as input to another function.
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[ "apple" 123 ./build.sh false (x: x*x) ]
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{ name = "Professor Paws"; age = 10; species = "cat"; formula = (x: x*2); }
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[#type-derivation]
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== Derivations
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A https://nix.dev/manual/nix/latest/store/derivation/[derivation]
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is an attribute set, but one which is a recipe for producing a Nix package.
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We'll learn how to write derivations in <<#derivations>>.
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For now, note that derivations are "first-class values",
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meaning that they can be treated like any other data type.
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For example, a derivation can be assigned to a variable, appear as an element in a list,
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be associated with a key in an attribute set, or be passed as input to a function.
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@ -7,6 +7,7 @@
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You can declare variables in Nix and assign values to them.
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[source]
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.Example
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....
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nix-repl> a = 7
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@ -24,6 +25,7 @@ Nix allows hyphens (`-`) in variable names,
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so `a-b` is interpreted as the name of a variable rather than a subtraction operation.
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[source]
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.Example
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....
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nix-repl> a-b
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error: undefined variable 'a-b'
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@ -52,6 +54,7 @@ effectively creating a new nested scope with each assignment.
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This makes it much easier to experiment in the REPL.
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[source]
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.Example
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....
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nix-repl> x = 1
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|
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|
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@ -4,6 +4,7 @@ A `with` expression is somewhat similar to a `let` expression,
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but it brings all of the associations in an attribute set into scope.
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[source]
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.Example
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....
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nix-repl> point = { x1 = 3; x2 = 2; }
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@ -18,6 +19,7 @@ Unlike a `let` expression, a variable defined inside a `with` expression will _n
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"shadow" an outer variable with the same name.
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[source]
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.Example
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....
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nix-repl> name = "Amy"
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@ -31,6 +33,7 @@ However, you can refer to the variable in the inner scope
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using the attribute selection operator (`.`).
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[source]
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.Example
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....
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nix-repl> with animal; "Hello, " + animal.name
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"Hello, Professor Paws"
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@ -1,3 +1,4 @@
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[#shebang]
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= Scripts
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You can use `nix shell` to run scripts in arbitrary languages, providing
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|
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