More examples.

examples.md

@@ -52,6 +52,7 @@ for plotting or rendering graphics.
 -   [Outer product](examples/outer-product.html)
 -   [Comparing arrays for equality](examples/array-equality.html)
 -   [Searching](examples/searching.html)
+-   [Option type](examples/option.html)
 -   [Computing histograms](examples/histograms.html)
 -   [Moving average](examples/moving-average.html)
 -   [Means](examples/means.html)
@@ -79,6 +80,7 @@ for plotting or rendering graphics.
 -   [Matching parentheses](examples/parens.html)
 -   [Evaluating polynomials](examples/polynomials.html)
 -   [Line fitting](examples/line-fitting.html)
+-   [Decoding MNIST data files](examples/mnist.html)
 
 # Automatic differentiation
 

examples/mnist.fut

@@ -0,0 +1,34 @@
+-- # Decoding MNIST data files
+--
+-- The [MNIST database of handwritten
+-- digits](https://yann.lecun.com/exdb/mnist/) is a common dataset for
+-- demonstrating machine learning techniques. It is distributed in the
+-- form of "label fields" and "image files", with a simple binary file
+-- format. The functions below decode this file format.
+
+-- First we need a utility function for decoding a four-element byte
+-- array as a big-endian integer.
+
+def decode_u32_be (w: [4]u8) =
+  (u32.u8 w[0] << 24)
+  | (u32.u8 w[1] << 16)
+  | (u32.u8 w[2] << 8)
+  | (u32.u8 w[3] << 0)
+
+-- Then we can define the decoding functions. These use assertions for
+-- error reporting. Some applications may prefer to use an [option
+-- type](opt.fut) instead.
+
+entry decode_label_file (s: []u8) : []i8 =
+  let magic = decode_u32_be (take 4 s)
+  let n = i64.u32 (decode_u32_be (take 4 (drop 4 s)))
+  in assert (magic==2049)
+            (map i8.u8 (take n (drop 8 s)))
+
+entry decode_image_file (s: []u8) : [][][]u8 =
+  let magic = decode_u32_be (take 4 s)
+  let n = i64.u32 (decode_u32_be (take 4 (drop 4 s)))
+  let rows = i64.u32 (decode_u32_be (take 4 (drop 8 s)))
+  let columns = i64.u32 (decode_u32_be (take 4 (drop 12 s)))
+  let get_img i = unflatten (take (rows*columns) (drop (16+i*rows*columns) s))
+  in assert (magic==2051) (tabulate n get_img)

examples/opt.fut

@@ -0,0 +1,16 @@
+-- # Option types
+--
+-- Futhark does not predefine an option type, but we can define on
+-- ourselves.
+
+type opt 'a = #some a | #none
+
+-- It can be used for example to define a safe indexing function.
+
+def index_maybe [n] 't (x: [n]t) (i: i64) : opt t =
+  if i >= 0 && i < n then #some x[i] else #none
+
+-- ## See also
+--
+-- The [containers](https://github.com/diku-dk/containers) package,
+-- which defines an option type and various helper functions.

examples/sum-types.fut

@@ -60,3 +60,7 @@ type dir_2d = #left | #right | #up | #down
 -- anonymous type that we have not even bothered naming?  The type
 -- checker will complain in these cases, and we can solve the problem
 -- by adding, for example, a return type annotation to the function.
+
+-- ## See also
+--
+-- [Option types](opt.fut).